Master studies (Patreon)

Lately I've been feeling like I'm getting stuck with my artistic abilities, so, instead of letting the art block arrive and get me too frustrated to draw I decided more practice needs to be done. That practice would be in the form of "master studies" using quick speedpaints or warm ups to recreate works of artists I admire in a way to figure out the way they do their pieces.

Question: ¿Which start should I start studying?