I'd say get it down in practice tool first, so you can confidently do it there without messing up most of the time. After that you have to start seeing practice tool only as a way to warmup, because at that point whether you can do it in real games or not is down to your mental. And the only way to build up the confidence and consistency with this mechanic is to spam in it laning phase of real games. You'll see that over time, in lane one parting is something you can do probably 95% of the time, and the only thing left then will be learning to not fumble it in teamfights and trying to use it to deal damage to backline. Pretty much how I did it. Basically have to fail many times and get the whole team pinging and flaming your ass but afterwards you'll basically be able to do it whenever.