In order to perform human flag, you will need a pull-up bar with a vertical support. Using your one hand, grab the overhead bar while pressing into the vertical pole with opposite hand to press the vertical pole that supports the bar.
Be sure to keep both arms straight with your lats and shoulders engaged.
Raise your feet as you press into the support bar with your bottom hand, while pulling from your upper arm.
Avoid bending your elbows when you stretch your whole body while standing at an angle of about 45 degrees to the floor.