on a previous guide i read that you could just get it by doing all commissions, that was not the case for me.
However i got a bug in which i had to uninstall and reinstall for the achievement to pop.
10 to 20 full size canvases with a big brush should be more than enough, if you feel it should have popped sooner uninstall, you may be having the same problem as I had, then reinstall, pop a new canvas and give a few strokes of paint. Enjoy!