It is possible to hurt your neck with this one, so be careful. Practice your handstand to get balance and build up your neck muscles.

1) Do a handstand, so that your feet are in the air and you're using both your hands for support. Practice this and make sure you can stay up for a long time. Then try moving around in a circle by "walking" on your hands.
2) Move you legs as far apart as you're comfortable with. Twist your hips in the opposite direction that you want to spin in.
3) Twist your hips back in the direction you want to spin in and bring your legs together in a circular motion to get momentum. You can also push off with your hands to help with speed.
4) This should give you anywhere from half a rotation to a full rotation at first. Once your spinning your hands come off the floor and you use them for balance.
5) You could leave it at that, but once you've got to a full rotation and you want to keep going, you bring your hands down and push off again.

A smooth surface is essential. You can wear a hat if you don't want to mess with your head.