Take an underhand grip on the the pull-up bar just inside of shoulder-width and let your body hang down. Creating a small reverse curvature in the lower back before commencing the movement will help to keep the focus on the target muscle rather than carrying all of your body weight through your biceps. This is the starting position.
Pull the chest up through your arms to the maximum height you can, closing the angle at the elbows and exhaling as you do. The elbows remain close to the body. Minimise momentum and swing throughout the movement to maintain focus on the correct muscle group.
Squeeze for a brief second at the top of the movement before lowering your body back to the starting position, inhaling as you do.