Start by grabbing the bar with your thumb wrapped around and the palms either facing towards or away from you. Begin the chin-up by squeezing your butt and abs then pull your ribcage down. The feet should be pulled tightly together with the toes pointed and legs extended in front of the body. Pull your shoulder blades together as you begin to strongly pull the elbows down and behind. Don’t “break” your neck by looking up in order to get your chin over the bar. Keep the spine neutral and abs engaged during the entire pull.
At the top of the movement pull hard enough that you can pause at the top for a duration of time under control. Lower under control and on the way down keep shoulders engaged and active throughout the entire movement This is especially important at the bottom, do not let your shoulders dis-engage.
Keep the entire middle tight and shoulders engaged throughout the movement.