Pin a suitable weight to support your body without removing all difficulty from the lift.
Take a wide grip on the the pull-up bar and pop one knee at a time up onto the support pad. Then let your body hang straight down. This is the starting position.
Bending from the elbows and exhaling as you do, pull the body up to the maximum height you can. Your torso should travel in a straight line vertically which closes the angle between your upper arm and the sides of your 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.
Dismount by taking one knee off the support pad then placing that foot on the ground. Keeping your hands on either the chin handles or the parallel bars, use your other knee and body weight to slowly let the support pad come back to its resting position. You can then bring the other knee off.