Start in a plank position with your legs parallel and your feet together on the seat of the rolling desk chair. Your hands should be directly under your shoulders, and your abdominal muscles engaged. If you don't have a rolling chair, put your feet on a small towel (if you're on a smooth floor) or on two sliders or paper plates (if you're on carpet).
Inhale to draw your knees to your chest, allowing the chair to roll in.
Exhale to push your feet back out to plank position. This counts as one rep.