every country will have its own procedures i'm sure.
here no costume is required, however, it is reccommended you dress nicely.
you & your partner go into a room where there should be the examiner only. perhaps the examiner will allow your teacher to be there as well. best to be aware of this now as for some people this is stressful.
the examiner will likely allow you to do a warm up routine.
when your ready you dance your routine. some examiners are strict about not talking with the examinee. it can be quite cold but remember deep down these people want you to succeed (if for nothing less than to encourage you to keep taking lessons)
if your partner is being examined as well the examiner will have you dance twice observing only one person at a time. you can decide which of you is tested first.
there is a charge. here examinees also pay for the hall. as we have a club with a 1000 members we set up a block period for testing to reduce costs for people.
IMPORTANTthe testing i'm talking about is under the international syllabus (ISTD). various other organizations set up testing which imhop is just designed to seperate you from your money. i had an experience this past summer with a lady who moved here from saskatchewan. she contacted me to inquire about our club. a senior, she insisted she was at the gold level as she'd won all sorts of gold medallions at competitions.
truth was she was a pre-bare-beginner who'd fallen prey to scam artist teachers who'd set up a "competition circuit" in various local towns (I understand this practise is common in florida and 60 minutes(?) did a story on it once:retirees spending $30,000+ a year on dance tours/lessons/competitions).
here's the link to the istd:
http://www.istd.org/OB