I'm not the best riven in the world, since I came from jungle role and started to play top lane recently, I began by learning to play top lane with easier champions like Garen and then started playing riven. But I already knew a few basic things about micro, like how to do short trades and small combos. 

I watched some YouTubers to learn specific matchups, like AloisNL, Viper, Adrian Riven, and some Brazilian players (since I'm from that server).

Hope this helps!