Anchor an elastic band above the place where you'll place your top hand when performing the exercise. The anchor and the two spot you will grip need to be aligned vertically.
Wrap the band around the foot that will be on top, grab the support and get into a Human Flag position.
The band provides assistance in a convenient and affordable way, but where you anchor it can change a lot the assistance you are getting. To reduce the uncertainty, try to be consistent in the way you use the band.