Well first of all if you're having to ask for advice you may want to reconsider doing it. But if you're just asking for pointers for some added confidence that's alright. First things first, don't set it super early, make sure you wait until the end of the jump to set the trick. Of course if you're at the level to be trying doubles this should already be automatic, its just that under the added fear it may just happen. First flip is easy, one thing though is that it's probably better to tuck that one around quicker so you have room for adjustment on the second flip. Here's where you've gotta trust yourself, the first time you go onto the second flip it might feel kinda wierd, you just have to do it. Know that if you freak out and kick out it'll be much worse than if you pull the 2nd flip around. Wait till you can see the ground before you do the spin. Voila, you've done it!
Hope that helps. Definatly try it on water ramps first. Less scary so you'll be less likely to mess up, and if you land on your face it'll just sting for a few minutes instead of breaking your neck
ya I've been doing them on water ramps this summer.
As for progression on tramp the best I can think of is to do the trick in 2 bounces right in a row. Maybee do 1 1/4 flips to your back so you get the feeling of going past 1 flip