Get someone to serve short, short, short to the FH side. Ideally the serve has the kind of sidespin you would get from a BH serve, reverse pendulum serve and a hook or tomahawk serve and whether it is topspin, straight side or backspin is varied. So, sidespin that curves towards the FH side and varying amounts of back or topspin with the sidespin.
They keep serving short to your FH (ideally very short) and you keep flipping. You do several 100 of that every day, and it becomes a good shot.
Practice, practice, practice.
Why is reverse spin good for practicing this? It is harder to flick a serve with the sidespin that curves towards your FH side. When you are solid with that, flicking vs the sidespin that curves towards the BH side with the FH should be pretty easy. Also, it is not so easy to serve a ball that is curving towards the BH in a way where it is short and on the FH side. But in the end, you want to be able to flick either. So, it getting changed up is good too. The real key is, you can't expect to be really good at a shot that you have not done over and over and over again.