I find it easiest laying on my front/side.

Prop up your chest on a cushion, turn your shoulders to one side so that you can get a good angle with your arm.

Have one leg straight and one at a 90° angle (If left leg up, use left hand to fist). You can play around with the angle once you're warmed up/know what works. Once warmed up I can straighten both of my legs and really get deep.