To do a double flip, put your feet in the same position as in a kickflip, but your leading foot should be hanging off the board slightly more and should be further back than for a kickflip. Pop a kickflip and as you slide your leading foot forward slide it off your board with a very fast motion. This should make the board spin faster than a kickflip. While it's spinning bring your leading foot back. Once it has spun around twice, smack the board down with both feet.
Hints: This one may take a while until you can get it to spin around twice perfectly, then you will be sorted

Trick contributed by: www.skateboarduk.com