Stiffen your body out so that it is rigid and straight, as in the pictures. Then, keeping your elbows up and upper back slightly arched, pull yourself up towards the bar. Again, think of pulling your shoulder blades together and pushing your chest forward. At the top of the rep, as shown in the pic on the right, squeeze shoulder blades together and hold yourself there for a second or two. Chest should be pushed forward and upper back slightly arched.
Elbows are out.
Squeeze your glutes a little to keep your body rigid, and keep abs tight. To add resistance, increase the height of your feet with a step, weight plate or bench. Once you get really good at it, put a plate on your tummy or across your hips.