The double heel flip.
The first thing you need to know about doing the double heel flip is you need to know how to do a regular heel flip.
The next thing you need to do is hop on your board in your riding stance.
OK the next thing is you need to place your feet, the best foot placement is to put your back foot right on the tip of the tail, with your toes right on the edge of your board.
And you want to put your front foot either right on the front bolts or a little behind them, and you want your toes to stick over the side of the board.
You really want to have your front foots heel almost in the center of the board.
Next you want to Ollie and kick out like your going to do a heel flip  but you really want to kick extra hard to give the board more power or momentum because it has to spin twice.
Also a good tip when doing the double heel flip is when you ollie and kick out bring your knees up close to your chest, this will get your feet out of the way so the board can spin freely.
One of the biggest problems people have when doing double flip tricks or big spins is their feet get in the way of the board spinning or flipping over, so you can eliminate that problem by raising your feet up a little higher than normal.
Also if your having trouble getting the board to spin twice try scooting your front foot farther out (scoot your toes farther out on the side of the board).
So go out and try it out.