Once you have passed the test, you should observe a tribunal to see how a hearing works in practice.
Most hearings in the employment tribunal are public. You can simply attend any hearing centre and choose a tribunal to watch.
If you can, try to observe a short unfair dismissal or wages claim, since that will be similar to the sort of case that you can expect at FRU.
When you arrive you should explain to the clerk that you are a FRU volunteer who would like to observe a hearing. They will try to find a suitable case where the appellant is happy for you attend.
Try to see an ESA or PIP appeal.