Grab the bar with an overhand grip. Jump into a ball pulling yourself in a skin-the-cat (hanging using the rings and rotating the entire body through) manner.
As a starting position, get your legs over to the other side of the bar.
Straighten the whole body so that you are hanging almost completely upside down with your legs above the bar and your torso below.
Now start to lower yourself carefully one inch at a time while pitching your chest forward. The key point is to get your body parallel to the floor with your hips directly under the bar.
Remember to contract your lower back, abs, glutes, and hamstrings while performing this move. Your arms make only one small part of the equation.