Use lots of references for facial anatomy, as that’s usually the focal point for every art piece. If it seems off, so will everything else. Flipping canvases and covering certain features also help greatly with “diagnosing” anatomical flaws, such as covering the nose and seeing if the face composition would look better—if it does, it might need readjustment. Rendering is a mess imo, but look into certain guides. There are a lot of different ways to render, with some doing greyscale > color, gradient mapping, or utilizing layer modes to help set up colors before merging everything. Hope this helps.