in general you can just show up. but I recommend an appointment so that you will be viewing the collection with a guide - your SA and you get more attention.
Most of the trunk shows require 50% deposit. (Balenciaga boutique+barneys). But Barneys doesn't always call the customer when items come in so you have to stay on top of it and call often when the season comes.
Balenciaga trunk shows are always quite nice. You don't have to put downpayment if the store orders the particular item in your size. But you never know if there is a waitlist already during the trunk show (every SA has their own list so say if four people decide to wait in line and they only ordered two, then there is a chance that you won't get it although you are the first on your SA's list - then there will be a fight over the popular items - good luck). But if you pay 50% down then there will be a piece of garment made FOR you so it's guaranteed. -- the way it operates is indeed slightly different from Barneys.
Also at Balenciaga you can order the clothes shown on runway but the store didn't buy (or didn't buy your size). Some times different fabric and colors are also offered, which is really nice. You'll be able to get something that is unique.
I haven't been to a Bergdorf trunk show yet so I'm not sure...
at Marc Jacobs it's more like a wishlist. no downpayment needed. of course it's not guaranteed then. But you have less obligation.
then just dress nicely and show up. enjoy the food and drinks. Have a good time!