The human flag is one of the coolest calisthenic holds that you can do, but also one of the toughest. It may look easy but it probably looks easier than it is. Just lifting yourself up is one thing, but actually holding it is a whole different game.
It can become frustrating if you are trying to master the hold but keep hitting a dead end and not progressing. Chris Heria from Thenx gives a really great and in depth tutorial on how to progress and ultimately master the human flag.