What your mini pupillage will entail depends on the set itself, and to some degree the barrister that you are with, so it's difficult to be precise. But in the main a mini pupillage usually entails going to court with barristers and watching what they do. That might be with one barrister throughout the week or, more often, different barristers. If the chambers does different areas of law, you might be with different barristers in different practice areas through the week. You'll obviously talk to them about the cases you see and have an opportunity to ask questions, but in the main you'll be a spectator. That's not always the case. If you're doing an assessed mini pupillage there will be work assigned to you that will be assessed, but only a small minority of mini pupillages are assessed and you'd know if your mini pupillage was. Outside of assessed mini pupillages, there is always the chance that you'll do more than simply spectating, whether that is having an opportunity to read papers in advance of a case, or actually doing something more practical such as taking notes for the barrister in court, or even trying your hand at some drafting or similar, though the latter isn't likely unless you've done the Bar course or otherwise the barrister has reason to think that you'll be able to do it. Either way, mini pupillages are not a test. They are an opportunity for you to gain experience and insight into what a barrister does. Even if you're given some actual work to do, which in the round isn't likely, it's just a matter of doing your best with it and asking questions as you go if you need to. Even when talking about a case, and if a barrister asks you questions about what you've seen or read, don't feel like there is any pressure on you to come up with the 'right' answers or to show particular insight. Mini pupillages should be a relatively safe space to learn. It's not something where there will be any expectation on you to perform.
So all of that said, I'd prepare by having a look through the chambers website at the sort of work that they do, as well as any recent articles or similar that have been posted. Most barristers will approach your mini pupillage on the basis that you know absolutely nothing about the law or the chambers, so again, don't feel any pressure with this. But it might just help you to gain a little bit of familiarity with the chambers before you do the mini pupillage itself. In the round, though, it's something that you should approach with an open mind and you should be looking to learn, not to prove anything.