If any of you virgin carbooters are up...
Take a friend or DP if he will be any use.
Take a float, you first customers will have £10 and £20 notes. Keep the money in pockets or a bum bag.
Do have a think about prices so that you don't sell your best next baby dress for 50p. Give a high price first, then come down if they haggle, if you start low you have no where to go.
Fill car now and put the table in last.
Carefull with pasteing tables they will collapse easily, if thats all you have put heavier items at each end and keep the light stuff in the middle.
If you have any clear plastic its handy for covering up if it rains it lets people still see what you have.
I use a parasol on a longer pole to keep me and the stuff in the middle of the table dry, watch that it is high enough not to poke eyes out. Set it up in a base in the middle behind the table for maximum protection 
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If its in a field take some plastic (bin bags) for setting out things on the ground under table and at the sides, so they don't get damp from the grass.
Take a seat, perching on the back of the car gets uncomfortable.
Take snacks so you don't spend all your takings at the burger van, but a bacon roll mid morning is compulsory.
Go to bed you will have to be up early, earlier than they say as people will be setting up at least 3hrs before opening time.