Ride directly at the ramp you want to Indy off of. When you reach the top, ollie as high as you can. The higher you ollie, the more time you will have to pull off the indy.
While you are in the air, simply reach down with your back hand and grab the board on the toe edge, right between your feet.
The key is to bend your knees very deep when you ollie, and keep them bent for the Indy grab. The more you bend your knees, the easier the Indy will be.
Now, hold the Indy for as long as you can, but make sure you let go before you come back down!! Most skaters barely grab their boards at all when they perform grab tricks, and if you're having fun with that, then go for it. But, it looks cooler if you can get more hang time and hold onto the board a little longer. That will take practice, and getting your ollie off of the ramp higher.
When you land, you want to make sure that your hand isn't under the board. You will be landing fakie, just like in a vert ollie, and the only thing left to do is ride away!
