My first Instructable! I figured this out one day and it has been my secret, but I want to share it with you all. It's not very good...
Step 1: Starting out
Fold the square piece of paper into a triangle and unfold, repeat on other side. Fold all the points on the square in to the middle. Just follow the pictures here.
Step 2: Triangle to diamond
Fold the square into a triangle, like in picture 1, then fold up the sides (pictures 2 & 3) and repeat on the other side. Holding just near the middle lift the bottom up (picture 4), then move it up to the top of the triangle and push flat. Repeat on the other side and it should look like picture 7.
Step 3: Now the harder bit...
Fold one side of the diamond in (pictures 2 & 3) and repeat on other side. Fold the top down (pictures 4 & 5), unfold and repeat on the other side. Then (pictures 6-9), hold the square down and just fold up easing it up on the way. In the end it should look like picture 9.
Step 4: Finishing up
Fold down the top flaps (picture 1). Gripping the sides, pull them out and then put your finger in the centre of the four pointed star and push it up (push the center of picture three). When it has inverted (been pushed through) poke around the middle a little until it fully puffs up.