Open your legs, and bend your knees slightly. Your feet should be planted roughly the same distance apart as your shoulders, and your back should be straight. Keep your posture loose. You won't be standing still for very long. Start with your right foot. Lift it straight up a little ways off the ground and return it, finishing with a small skip-step backward.  To perform the skip-step, let your foot hit the ground and bounce back up slightly, kicking it a few inches back rather than popping it up again. To keep your balance, you will have to hop a little bit, which is also a part of the dance. Practice going back and forth from your right foot to your left and back again, until you feel like you can keep up a rhythm easily. Now that you're comfortable with the movement, you'll need to learn a simple pattern. The dance is performed in sets of four steps that alternate back and forth.  The pattern is as follows: right foot, left foot, right foot, right foot, followed by the opposite. This means you step and skip once with your leading foot, once with the other foot, and then twice more with the leading foot. After that, switch your leading foot and repeat.  On the last two steps in each set, you'll find it's difficult to skip-step, since it naturally puts your weight onto your other foot. Do what Psy does and just keep your foot light, instead of taking a full skip-step on these steps. Don't kick back on them. Practice this RLRR, LRLL pattern until you can keep a rhythm doing it.

Summary:
Get a proper stance. Learn the steps. Learn the pattern.