Lie down, roll slightly onto your right side, and bend your right leg in. Rest your right foot flat and firmly on the ground.
Lift your left leg, cross it over your right, and rest the ankle of that foot against the top of your thigh. Move it to wherever it's most comfortable.
Shoulder your rifle, but maintain as much contact with the ground as possible on your right side, from the shoulder out to your elbow. Your right arm should be perpendicular to your chest.
Press your left elbow into the bend of your left leg, just below your knee, and rest your forearm against your left shin. Solidify the position by tightening your left hand across the rifle's fore-end, just under the barrel. Center the crosshair on the squirrel's head, squeeze the trigger, and hope that no one is around to watch you untangle yourself from this predicament.