Basically, As you jump in the air, pull one leg behind you, and kick the other infront as far as you can. The further you can stretch the better it is. Flexibility and timing is key.
This is actually one of my favorite tricks. It looks great in photos, and is easy to do once you've mastered the basics, and can maintain balance in air. To get the best split kicks, I've found that varying what you do with your arms in air helps a lot. Bring one arm up, one back, ect. Variety is key in making the entire trick nice.