Just copy and pasting my post from another thread
" Just try it on a small jump. You can probably 270 just on the ground. Just hit a small roll in the trail and throw as hard as you can, should be able to start poppin 3 off nothing. You have those 2 edges to grip, set up like you would spin on a trampoline, just grip and rip that shit. Switch 3s will take a little more time but start small and work up. Even if your park doesn't build small jumps(which most do) start on cut ups, rollers, build a jump in your yard or local sledding hill etc.
Most peoples biggest problem with 3's is the jump. They try and jump the 360. You want to bend your knees and really just throw the spin. If you jump upward and bring your arms up in that motion you just won't be able to get the spin. And even if you somehow manage to get a 3 around like that, 5's 7's etc are out of the question. You can almost bend your knees in a little bit like you're going to wedge. Wind that shit up and just fucking whip them. Go straight side to side though. Don't throw your arms up in the air at all, stay even height in the pre wind and the follow through.
As your arms get to the front let them almost pull your head with them. Just like the arms keep your head level. Once your arms and head are turned all the way, hold them there. Keep your head level and looking for the landing.
From there to progress to 5's just keep your head turned. The hardest thing about going from 3's to 5s is keeping your head turned, don't peel it off to look at the landing from 270, if you're eyes focus on that you'll stop your spin. Once you get 5s, everythig after that is pretty much just going bigger. "