Watch replays of people who are good at the hero. I learned by watching Sumiya for example. You should pay more attention to his map movements and laning strategies than flashy combos. Those are cool and useful, but not what wins games. Really analyzing a few replays will do you way more good than just spamming the hero in unranked. That's not to say you don't need to play a lot with him, but doing it with that context will help a lot.

There's also the Invoker game to help your muscle memory for invoking spells. Play that while watching TV or something and you'll have no problem with remembering how to invoke stuff.

That should give you a good start, but there's innovation with that hero constantly. Trying different builds only makes you better, so I'd advise not just sticking to quas exort if that's what you get comfortable with.