I started by drawing a construction line of the lady's face. Use a separate layer for each part of the drawing.
Next, sketch using a pencil like brush on Photoshop.
Now it's time to tone, use the colour picker to get the right tones for her face, don't be afraid to cut and paste sections of her face out so you can zoom right in there.
Make it as detailed as possible, it's lucky she has such distinctive lines and wrinkles.
