Start hanging on something low where your feet can easily touch the ground.
Grip the bar strongly with your entire hand. Can you feel the pinky finger edge of your palm hold as strongly as the thumb side of your hand?
Imagine that you are going to break the bar with the strength of your shoulders and grip.
Keep your ribs down and aligned over your pelvis.
How long can you hold?
Eventually move onto something high enough for your feet to dangle.