The 1990
[1] Practice doing a handstand and staying up for a long time. This will help you with your balance and with the 1990. If you can't do a handstand, try using a wal for support. If you're using the wall method, try balancing on just one hand once you've got hte two handed approach down. Then try balancing on one hand for a few seconds.
[2] You've got your handstand sorted out, now you've got to get moving. You'll be using your legs to spin your body around. To practice, do a handstand and then practice twisting your legs around in a circle without actually spinning yet. Practice until you feel comfortable with moving your body with your legs and maintaining balance.
[3] Now you're ready to start the move. Do a handstand. Shift your weight to the hand you're not going to be spinning on. Spread your legs apart in the air. Now shift your weight to the hand you want to spin on. Make sure the hand you're balancing on is directly in line with your body, so that your head is directly above your hands.
[4] As you shift your weight to your spinning hand, bring your legs together and twist them at the same time as pushing off with your free hand.
[5] Spin on the lower part of your hand opposite your thumb, not on your whole hand. You can wear a glove if you want.
[6] The momentum from twisting your legs and pushing off will keep you going around for a while. Once you slow down, put your other arm down and twist your hips while pushing off with teh hand you started spinning on.