Find the hips by tilting the pelvis backward, and engage the abdominals by pushing the low back into the floor.
While maintaining the contraction of the abdominals, raise (or lower) your arms to the floor until your thumbs make contact with the floor.
Common Errors and Their Correction
Your rib cage may extend or your low back may arch. Avoid this by not attempting to touch the floor but only move as far as possible without your ribcage extending or low back arching.
Your shoulders may lift towards your ears or you may shrug. Avoid this by making sure your shoulder blades are pulled down and back into the floor.