To create a mug all I needed was at least 45min of uninterrupted time to layout the whole image and a good photo. Since the mugs were done on my own time, I decided to keep options minimal and select the photo myself - this, in hindsight, could have been a longer process where the person being drawn is involved.
Once the photo and the time was blocked, I used a Wacom tablet (if possible) to layout the edge of the face, the hair and the neck. Next came the eyes and the mouth. Finally, the details: nose, ears and facial key features. The key features were chosen to make the person 'look like themselves', not to represent every inch accurately. It's not a realistic representation exercise after all.
I have to say though that by the time I was doing my 12th or 13th mug, I could not help myself but be realistic, here's the last and best mug I made: Charlie's (see mug below on lilac background).