Note: This method is inspired from artist "Angel Ganev"'s techniques of portrait drawing.
- Select a good reference
- Paste your reference in the canvas twice, reduce opacity of one in the middle.
- Choose your preferred method of face construction, i use a mix of Loomis and Reily methos
- Draw the structure over the lowered opacity reference. Do not focus on details, break down simple shapes, and try to ignore any details like small crevices in chin, curvature of ears etc. Main goal is to capture the structure and get overall proportions right.
- Now redraw the same structure on the right. Try to match the proportions of the initial drawing.
- Compare the traced sketch and the untraced sketch. Make note of any areas of improvement, try to correct the sketch without tracing the image again.
Redo this process with multiple references. If you're just starting out, expect to use at least 30-50 references before seeing some improvement. Focus on getting the perspective, proportions, angles and line weight right.
Again, this is just how I learnt to sketch. Once you get this information stored well enough in your mental library, you'll start to sketch faces a lot easier without all the guidelines. This will also prove to be a base for understanding facial planes, which will improve your shading/coloring skills as well.
Share your tries in comments. Feel free to ask any questions and I can try to help you guys as well.