Lie face down. Put your elbows and forearms on the floor, so that the elbows were strictly under the shoulders, with the elbow forms a right angle.
Lift up your body and keep it straight, resting your toes on top of an Swiss ball. This is your starting position. It is of primal importance to keep your lower back straight. Keep your abs and glutes in constant tension with squeezed abs, to ensure correct position of pelvis, tucked tailbone and straighten core.
If your form breaks, stop, take a few minutes break and then repeat.