I had help from my mentor and it took me 2 or 3 weeks of practicing every chance I got before I even considered doing it in performance. I still could improve and I've been doing the move for about a year, but I think this is true with every move I do.
My biggest advise is to find someone who knows the move and have them critique you one on one, live if possible, via web cam if not live. This is what helped me more than anything. For one thing I learned it from Royal Road to Card Magic, and learning the pass from a book is hard, even if the book is one of the best. I had the mechanics wrong at first because I didn't understand the function of the right hand, and how the hands are supposed to cooperate, but luckily my mentor was able to correct me before I had the incorrect movement ingrained in my muscle memory.
I also don't suggest ever doing the move without cover, no matter how good you get. I always make sure that my spectators are looking away from my hands when I do the move, though I've done it with people looking at my hands and not been caught. If they're not looking away from my hands, I try to divide their attention between my hands and my presentation as much as possible.