Here are the steps:
1. Take both feet out of the stirrups.
This is really important, because it's easy for a foot to get caught otherwise.
2. Put the reins in your right hand.
3. Place both your hands on the horse's withers.
4. Lean forward so that your weight is over your hands.
5. Kick your right foot back and over your horses rump.
6. Push off the horse with your hands.
7. Land on both feet with your knees bent to absorb the impact.
8. If your horse was moving quickly, you may need to jog a few steps with him to keep your balance.
9. Continue to hold the reins so you have control over your horse.
To practice this, have a helper hold your horse at the halt and practice the movement a few times. Next have the helper walk your horse and practice the emergency dismount at the walk. Once you are comfortable try doing the movement on your own. Eventually you'll be able to do an emergency dismount at the trot and canter too.
Remember, typically it is better if you can stay with your horse and ride out situations, but an emergency dismount is a good tool to have just in case.