Hang from a bar with an underhand grip (= supinated), engage your scapulas back ( Shoulder retraction ) and down ( Shoulder depression ) and pull your elbows back.
Lift yourself up as high as possible, until your chin is above the bar.
Come back down with control until your arms are fully extended, but keeping your shoulders engaged.
Form tips
Keep a neutral neck: avoid Neck extension .
No jumping, kipping or swinging.
The Hollow body position works the Abs more while the Arch body position works the back more.