I’ve always been drawing since I was little, but in recent years(due to work and programming) its been difficult to find time for it. I only bought Betty’s book a couple of years ago but its teachings have made a drastic improvement in my skill. Beforehand I was more passionate than skilled…
The thing I have learned to do is to map(lightly) the image infront of me and then when I am convinced I’ve taken it as far as I can - I turn the drawing and reference upside down. Trust me - you’ll spot mistakes straight away!
Even after all that effort its still has mistakes - below his nose and the side of his face where the contour is too “bumpy”. It easy enough to say “oh, reference the center of the drawing a guide”, but you can reference other parts of the picture to help position nearby areas.
I suppose the best advice I could give is to explore your reference more than draw it. Thats what I shall do when I come to do my next picture.