Set-up: Set up with your hands slightly wider than shoulder width - so that the upper arms are tucked at 45-degree angle to the torso - and fingers pointing forward. With the toes together and hips up, establish proper upper body posture with a neutral back.
Lift: Press the chest off the floor until the arms are straight, and there is a straight line between your ankles, hips and shoulders. At the top of the push-up position, with your arms straight, push up - rounding your upper back and spreading your shoulder blades. To return the starting position, cue the descent by attempting to 'pull' yourself down to the floor with your shoulder blades.
Emphasis: Focus on the function of your shoulder blades at the end of the movement. Do not allow the hips or the lower back to pike upwards or to sag.