This is a partial answer to how you communicate your abilities during your pupillage. It also, I hope, goes some way to address the fears of the BVC students that their advocacy is ‘a trainwreck’ (a direct quote – and whoever originated that description is showing real talent in my book).
The job is popularly supposed to be all about advocacy. Every barrister dreads the mini-pupil who says something along the lines of “my Mum and Dad thought I’d be a brilliant barrister because I was so good at arguing/could talk the hind leg off a donkey”. In fact, the job is more about judgment. What you are trying to do is present a case in a way which will appeal to the tribunal. The appeal has to be on two fronts simultaneously – the case must be grounded in facts which the tribunal accept, and those facts must demonstrate a legal entitlement to what you seek.
Neither of those things happen because the advocate has developed a fine line in high falutin phrases which sound awfully good in front of the bedroom mirror/admiring partner/adoring parent. Nor do they happen because the advocate is motivated by indignation/anger/intense desire for justice. They happen because a good deal of thought has gone into the following:

• What the law is as regards the particular point
• What facts are necessary to bring the case within the law as it is
• What the tribunal is going to feel about the submission that the law is x
• How such feelings, if they are negative, might be overcome by presentation of the facts in a particular way
• How to elicit the facts so as to demonstrate that they support the legal conclusion you propound
• How to deal with facts that do not support that conclusion or go the other way

Your preparation should leave you with a roadmap to the result you want. Within that roadmap there will be areas which are conditional on particular facts being found. There should not be an area which is conditional on the law being a particular construct unless you are doing something quite out of the ordinary (most decisions are a restatement of the law to a particular situation – very few alter the law).
It follows that, before advocacy – in the sense of the glorious full-on assault on a witness beloved of soap operas, BVC students and the tabloid press – is relevant, a lot of work has already happened. If you don’t do that work then your advocacy will be aimless, sloppy and have all the effect of a vicious attack with a wet tissue. A great many (too many) barristers believe that ‘work’ means knowing the case backwards. They memorise the witness statements, their client’s proof of evidence, and their own cross-examination (often written out on 24 pieces of paper).
If such preparation works for you then by all means use it. My own view is that it tends to prevent speedy reactions to the unexpected. But do not believe that this is the work required. The best proponents of this method do not make that mistake. The work goes into a quite different exercise – that of asking ‘but what if she says x’. Here, ‘x’ is the unexpected or unanticipated answer and it is unanticipated or unexpected because you have not anticipated or expected it. Because advocacy is primarily about thought and, in particular, thought about the following:

• The effect of your case upon the witness and the tribunal. You have to stand back and ask yourself how what you are saying plays. For example, you may be able to cross-examine a victim of child-abuse into an admission that their abuser loved them. This is not a success. It simply demonstrates that the abuser (your client) manipulated and exploited a victim who was incapable of formulating appropriate emotional responses because of the abuse. The Judge will think your client wanted you to take this path and it will make matters worse. The way to deal with this is to state in simple terms the proposition that you are aiming for, and then ask if it helps. If you think that the above example makes things better then you have no judgement. In such a case you want the child to say that the victim did not abuse them. If you can’t get to that point then the roadmap takes you to a guilty verdict.
• How you best elicit a particular fact. In a simple road traffic accident it is quite often the case that one party apologises to another. A large number of clients – and an even larger number of claims handlers – believe that this is the killer punch. It is not. A fool rushes in and, unable to contain their pleasure at having the defining moment in front of them, says, “You apologised after the crash, didn’t you?” A: “yes”. Fool: “That is because (or – if they are both foolish and incompetent – “I put it to you that is because”) you knew it was your fault, didn’t you?” A: “no, it was because the lady said ‘now I’m going to be late picking my grandchild up’.” It would be better to ask the open question of whether anything was said and, when the witness says they apologised, ask why. There are only three answers to that question. They are the one above or some variant of it, the admission of fault (which is then all the stronger by being elicited by open question),  and the “I don’t rightly know”. The first and the last can then be confronted with gentle questions which suggest that this is an excuse arrived at after the event. If the witness remains adamant as to their explanation then the matter can be the subject of the following questions, “you apologised for some reason unconnected to your driving? Did my client apologise? Did you not ask her for an apology? Is it only after the event that you offer us this explanation? Can you explain why?”.  That may be the best you can do, and be sadly short of a Perry Mason moment, but it allows you to suggest that the apology does mean what you say and that the attempts to make it sound as if it is not are late and unconvincing because they do not accord with the party’s behaviour throughout. In other words, you have a factual platform for your legal case and you have offered the tribunal a reason to accept your factual version over the other side’s.
• The impression you convey. Are you properly dressed? Have you stood up straight and not fidgeted? All of this is important: it is about how you communicate someone else’s case. Your appearance is about facilitating that process: it isn’t the main event. You need to be smart, clean and conventional. Try and eliminate the personal tics.
• The way in which you deal with your case. You must ask one question at a time and only one. This is easily tested: either get a friend to take a note or record yourself making a submission or examining a witness. The note should flow without difficulty. If you are making more than one point, or asking more than one question, at a time or if the order is all over the place then you aren’t doing the job properly. How do you make sure there is an order to things? Also simple. Write down your case in the order in which the tribunal will need to deal with things to get the result you want. Ensure that this order is rational (it doesn’t have to be the only rational order but it must be as rational as any other approach). Then ask your questions and make your points in conformity to that same order. Normally, chronological order is simplest and best. In a very complex case it may be better to deal with things issue by issue. Other approaches are rarely, if ever, justified save that an issue raised by the tribunal should usually be dealt with straight away. Silks in the Court of Appeal can sometimes get away with ‘I’ll come to that logically when I have developed this point if your Lordship doesn’t mind’. But only sometimes – and you can’t ever get away with it so don’t try.
• How you will get on best with the tribunal. If you know your Judge then this requires an honest appraisal of how you get on with them in Court – and why. There was a Judge (now retired) who very plainly thought I was too clever by half. Of course, you are sitting back and gasping in astonishment, but it is true. I thought the Judge was an over-promoted incompetent who had replaced hard work with prejudice and thought with knee-jerk reactions. Unsurprisingly, I didn’t have a good record before that particular tribunal: until I grew up and realised that, even if I was right, I was wrong. The job demanded that I adapted myself to the Judge. That, in turn, demanded that I worked on the basis that the Judge was right about me – unbelievable though that was.  I moderated my behaviour and the results improved. The unsettling conclusion that the judge might have been at least partially correct in his assessment was also valuable.
• What the reaction might be to a particular question or point. You need to have considered the other side of the coin in order to have an answer to it. I know people who essentially have a notebook with ‘if x then go to y; if a then go to d’ written out. I tend to keep that in my head. But any competent advocate has done the exercise.

When you have done that, you are ready to speak. If you are trying to be an advocate without having done this then, unless you are amazingly lucky, you will have a trainwreck.
How you put it all across is the easy bit. You speak slowly enough to be clear without being maddening. You speak loudly enough to be heard without being in breach of the Health and Safety legislation. You use language appropriately – the number of aspirants who think that pompous language is a good thing is staggering. I have concluded, over the years, that the use of language is a defensive thing aimed at demonstrating intellect and capability or ‘weight’, both of which things the aspirant thinks (often with good reason) that they lack. Don’t do it – it sounds false and adds nothing. Speak sensibly and only use long words if it really sounds like you. If you want to improve your vocabulary and use of English then read good literature, don’t learn a word a day and use it because it’s got six syllables. Sound reasonable, not outraged – less is more. If the other side really are arguing something outrageous the tribunal can normally spot it without you drawing yourself up to your full height and sounding like a third-rate actor auditioning for a fifth-rate production of a revenge tragedy.
Show a mastery of the facts. That is done by appropriately placed allusion to the evidence and the facts and knowing where those are to be found. The aim is that the tribunal should have absolute confidence in your ability to direct them to the relevant document or passage. It does not need a long exegesis on the four previous drafts – that’s showing off.
Pare it down. Less is more. Lord Salmon (of Sandwich – let no one tell you that Judges lack a sense of humour) once said that no case, however difficult, involved more than two key issues. Most have one. So how long do you think you need to talk for? If the Judge isn’t sure, but thinks you might be right, she can always ask you a question.
Never accept a departure from that standard. Barristers who say “it’s just a bail application/DJ’s appointment” are unfit for purpose. So are those who would rather go out the night before than get to grips with their work. So are those who say it’s all too hard for them because they aren’t really familiar with the law in the area. There are no excuses – there is only professional competence or professional incompetence. If your ego requires you to make excuses for yourself then do something else.
People write books on advocacy and this is necessarily short. But I hope I have covered the basics. At the beginning it is difficult. You don’t do well and you feel desperate and certain that you can’t do it at all. But we are talking about a skill. It is as foolish to think that you can master it in months as it is to believe that you can play Mozart’s Piano Concerto No 16 (well worth a listen) when you’ve had lessons for a year. It is even more foolish to suggest that you are the finished article when you begin tenancy – for if you are then it is only because you lack the capacity to learn.
The great thing for you is that the type of preparation I have talked about can be demonstrated to people who do not see you on your feet. Talking through likely issues in submissions or cross-examination can be relevant at any stage of the case, and can impress professional and lay clients at an early stage. It shows your supervisor or assessor that you are thinking like a barrister. And they might even help you do it.
One last thing: be courteous. Sneering comments about the rick that other people have made of the case serve only to cheapen you and to make more experienced heads wonder why you are so anxious to big yourself up or (if you have made common cause with the object of your comments) what you did to ensure the case was lost. Advocacy is lonely and judges are not always persuadable. Judgment is exercised on the hoof and many is the time that a rewind button would be of help. The test of the advocate is how many times they make the same mistake – not how unpleasantly they can critique other people. Advocacy is about sustained achievement, not about yesterday’s result. We all get nose-enders and, hopefully, we can laugh at them later. But those stories always come best from those to whom they happened – letting the robing room or common room know about someone else’s misfortune is the act of someone who lacks confidence in their own ability, or simply likes to promote laughter at the expense of others.