No exercise engages the scapula, quite like the push up. For strengthening the arms, shoulders, and body core, nothing beats this traditional exercise. The push up also engages the serratus anterior muscle along your rib cage, the same muscle targeted in traditional standing postures with extended arms.
Lie flat on your stomach with your head and nose touching the floor. Place your hands next to your shoulder blades, with your thumbs touching your shoulders.
Exhale as you push away from the floor. Keep the Wuji Posture alignments thoughout the push.
Inhale as you lower back to the floor.
A set of 25 Push Ups is a good place to start, and you do not need more than that, if you are practicing them daily. However, if you are intent on using internal martial art practice as a boxer, then you should do 125 Push Ups as a single set. Anything less does not develop the scapula and serratus anterior muscle sufficiently for developing power in a strike.