You can either hang from a pullup bar or situate your arms inside straps. This instruction will be for the traditional shoulder hang.
Your arms are just outside shoulder-width with your shoulders braced.
You are not dangling from the bar.
Image bringing your armpits forward as you grab the bar.
Have your pelvis underneath your ribcage.
Don't let your low back arch.
Feet and legs are together with knees slightly bent.
Imagine bring your toes to your face.
Your pelvis should "curl" slightly. The purpose of the rectus abdominus muscles is to flex the trunk.
Create the mind-muscle connection of your abs contracting like a bicep curl.
You don't want the torso rigid like you are bracing for a squat.
Negate utilizing your quads and hip flexors to raise legs up.
Reverse the path on the descent, uncurling your torso from the top.
Keep the strong core position on the descent, don't let your low back arch.
This will disconnect your core from your lower half.
This movement takes time to develop. You have to develop the strength and stability in your shoulders to properly hold yourself. Then one has to practice the movement pattern to properly activate your core while executing the movement.
Don't SWING YOUR LEGS UP!
A great introduction to this exercise is raising your knees up to your abs. This will aid in building the necessary core strength and shoulder stability.