Set up in a high plank position with your hands under your shoulders and your feet together. Your body should be in a nice straight line from head to your heels.
Without bending your elbows or dropping your hips, pinch your shoulder blades together and press your chest out. Do not tuck your chin or jut your head forward. Also do not let your core wiggle or your elbows bend to try to increase your range of motion.
You are not doing a push up. This is a very small range of motion where you are simply focused on pinching your shoulder blades together. Just pinch your shoulder blades together and then relax or even round your back up out of it (rounding your back at the top is another variation but can be very useful for correcting certain push up problems).
Keep your core tight as you pinch your shoulder blades together.