Take one large step backward with one foot. Bend forward at the hips to make your abdomen just barely touch the thigh of the leg in front of you.
Reach your arms and head forward in front of you to counterbalance your body as you bend forward and step backward.
While you fall keep your chin tucked into your chest to prevent neck injury or strain.
As you are falling turn your body a little bit so you don't hurt your tailbone. Roll down onto your side as you land on your bottom. Basically you are falling down into a sitting position.
As you land put your hands out to your side and onto the floor without putting pressure on your hands.
Practice falling over and over. With practice you will be able to perform this backwards fall so that it looks real.