My best method of landing Charm is to never use it in situations you are not garunteed to land it (in these situations there is no reason not to use if and you should if you KNOW it will land. One of many examples in this senario unclude a stunned or rooted or hard cc'd enemy. Yeah, go ahead and Charm) Or, use your Spirit Rush to immediately place yourself closer to them and off to the side (a diagnol dash) and either use it immediately, or q + w and 2nd Spirit Rush, and perform the Charm in this window (Dashing parrallel with them, effectively chasing them)
What this does is reduce the amount of times you actually use Charm, but greatly improves the accuracy and frequency with which you land it. Sure you might be missing out on a few Charms you may otherwise not attempt, but you'll get a greater grasp of the move and at the same time train yourself to play Ahri in a style that doesn't mainly involve her Charm. It's a big part of her kit, of course. But if you can be good, and sufficient with Ahri without Charm, then actually using and landing Charm will make you a better Ahri player. I hope anything I have said at all is in any way remotely useful!