Rest your elbows on a large Swiss ball and get yourself into a proper plank position. This means that your arms should be bent at a 90 degree angle and your entire body should form a straight line from your head to your ankles.
With your abs braced and glutes contracted, simply move your forearms in a small circular motion while keeping the rest of your body stationary.
Perform 2-4 rotations in one direction, and then reverse directions by performing 2-4 the other way, continuing for the prescribed number of reps.