Not in this order, but make sure you do most, if not all of the following:
Make sure your bike is in full working order as well as de-greased, re oiled and cleaned. (The day before)
Write a list of all the stuff you need, tick off as you pack it in the car/kit bag. That’ll stop you forgetting something. (so: spare tubes, pump or C02, warming riding kit, cold riding kit, allen/hex keys, etc)
Get to the circuit nice and early after a good breakfast (Porridge, Muesli etc).
Get some warm kit on, and go and ride the circuit. If you struggle on any sections re-ride them, and try to pick out the best lines.
Once you’ve done that go and chill in the car or whatever, then no later than 45mins before your race starts, get a good warm-up.
Warm-up should ideally include some sprints, and spinning through the gears. This will get your legs nice and loose before the off. You don’t have to stretch off, but it can help. So include some basic 30sec hold stretches if you get chance.
Unless, they’re gridding from a previous race. Try and get to the start about 15minutes before, so you get a good place on the start grid. (Nothing worse than being at the back!)
Oh, and good luck! Enjoy.
Ciao.