Step 1. Find a pair of low grip shoes you could try to do it in your socks to start off with.
Step 2. Make sure that the ground you use to practise to moonwalk on is also not too grippy, try and find a polished floor.
Step 3. Stand with both feet close to each other, left foot slightly ahead of the right (toes of right should be in line with half the left foot)
Step 4. Now raise the heel of the right foot so that you are standing on the front of the right foot as if you are taking a step. The left foot must stay where it is (take care not to move it).
Step 5. As you lower the heel of the right foot, lean all your weight on the right foot, and drag back the left foot to so that its toes are in line with the heel of the right foot. The left foot's heel must be slightly off the ground at this stage. As you drag back, do not push down on the left foot at all or it will not glide. Make sure as you lower the heel of the right foot (slowly) the left moves at an equal speed. This will need lots of practice to master the right speed.
Step 6. Keep practicing up to the above steps until you can make the movement subconsciously without any difficulty.
Step 7. Once you have mastered that, "kick" outwards with the left foot, but although not quite touching the ground, make it look as if it is touching. Move it out a foot-size's worth away from the toes of the right. No part of the left foot should be raised higher than another.
Step 8. After you make your left foot move so it is at the starting position, lift up the heel once more of the right foot. Make sure the left leg is bent at the knee. Now repeat step 5. Keep practicing until you have the whole thing figured out, and it has been verified by others, and you feel quite comfortable with it. You should eventually get that gravity-defying effect and you'll MoonWalk like the pro.
Step 9. Once you've figured it out for the right leg bending, switch legs, and try the same with the other foot. Lift heel of left, lower left as you glide right back. Left still on the ground, throw out right foot, lift up heel of left foot, and once again drag right foot back as left heel is lowered.