Shave legs no later than 24hrs before and exfoliate face legs and arms well for a good few days so no patches. Moisturise well. Do bikini line.
On day remove all traces of perfume, makeup and deodorant and tie hair back.
Go there and leave wearing loose dark clothing. Loose harem pants and a loose vest was best I found and a dark string bikini top.
Try and dry it as much as possible before getting dressed or leaving. They will have a fan or hairdryer to do that. I used to have them done in Sydney in high summer and keeping dry for 2 hours on public transport was impossible!!
Go darker than you think. Spray tans are remarkably good. Go for one layer in the face to keep it natural.
Your tanner will chat to you when you get there about colour. Go darker than you think you need. You will be led to a cubicle and told to undress and put on some very silly string paper knickers and stick some things to your feet so you don't get orange soles and told to stand in a little tent. Tanner will come in and load gun and give you instructions while spraying: arm out, head up, turn 90 degrees, bend leg, hold boob out the way etc. Takes about 10 minutes. She'll hairdryer you and leave you to get dressed.
Don't exfoliate afterwards but moisturise like a madman for 7 days and it should last nicely.
Done.