The above solutions are good, but I found that instead of 1 on 1 it's best to do it on a breakaway with 2 players as the flair shot is very weak.
The key is to do a cross pass or throughball when the goalie is rushing you. Then when you have an empty net you should hold LT (I held both LT and RB) and tap the B button - do not hold it, just a quick tap.
See my clip below.