You line up for said event- while in line, staff will group you up into your party. There, you can discuss which of the stations (job) everyone will play as well as your strategy for the fight. There is always going to be WAR, PLD, SCH, and WHM, DPS are usually DRG, MNK, BLM and BRD though I suppose there's a chance they might be throwing NIN into the mix as a special for FanFest.

The characters are premade, so you don't get to choose race, you're only choice is to pick the job you can play best while also allowing other people in your group to pick a job they are knowledgeable/comfortable with. Once you sit down at your station you'll get a few minutes to make changes to your bars to set it up how you like. It's never enough time, and you'll never get the exact setup you want or use at home. You're best option for a comfortable UI is to use the default UI now at home and get a good feel for where all of your actions are so that at FanFest, it seems more familiar to you.