basically you can start by doing any trick you can imagine, as long as you can legitamently land with your board oriented... well, like you are gonna be able to land it solid enough to ride away. at which point right before you do land, you pull both triggers, and you'll see your guy start to go for the double grab looking like a cannon ball. he doesnt get all the way to the grab (if he does, he'll fall, and you didnt do it right), but make it so he's reaching down for the board RIGHT before you hit the ground (mid reach). its a pretty small margin for doing it or just demolishing yourself upon impact, but if you're doing it too late, you might even see your guy land on his board and then just slip off with a foot and he falls. small window you just have to time it correctly just RIGHT before impact of landing.
and the ledge / drop im talking about is the one in the intro that ALI BU-ALAH (spelling) drops, it's the one that you're SUPPOSED to go hella slow, and just drop and land on a small tranny at the bottom. its got a railing on the left hand side of it right next to the launch spot. if you go to slappys shop, go up the rampish hill pathway designated there and then travel across the street you should see exactly what im talking about. i only mention that place because you cannot physically land it any other way if you wanna take any amount of speed past basically rolling over the edge. you can boost huge though. this is where i first was introduced to doing it at, the huge ledge
this works on xbox and ps3 versions. good luck and hit me up if you need