Listen to the music for a few minutes. Count the beats in each bar; there should be four. You'll be doing four distinct dance steps, one on each beat, and then repeating the steps.
Stand with your feet about a hips'-width apart, and bend your elbows so that your hands are up by your waist. Keep your hands in loose fists.
On Beat 1: Bring your left elbow forward, across your body, and slightly downward. At the same time, bring your right knee up toward your left elbow. The knee doesn't have to touch the elbow; it'll be closer or farther away depending on how vigorously you're skanking.
On Beat 2: Bring your left elbow and right knee back to the starting position.
On Beat 3: Do the opposite of what you just did in Step 2. Bring your right elbow forward, across your body, and slightly downward. At the same time, bring your left knee up toward your right elbow.
On Beat 4: Bring your right elbow and left knee back to the starting position.
Repeat steps 3 through 6, and keep repeating till you're tired. Now you're skanking!