At first I was wondering whether to go for a caricature or a straight up portrait, but I decided on the latter since I felt I needed more practice in that area. To make it a bit more fun I took a reference picture where I made a somewhat funny face.
Me operating a narrowboat in a lock on the British canals.
After the block-in was completed I started to think about the form, edges and the in-between values. I tried to see the face for my minds eye in 3D, thinking about things like bounce light and translucency while staying within the extremes of my value scale. The highlights and the darkest darks in my painting are never absolute white or absolute black - far from it. This adds a lot to the realism effect.
At all times I try not to think about exactly what I'm drawing (except for maybe the latest stages in the painting). What I mean is that while drawing a nose don't think "now I'm drawing a nose", but rather just try to focus on the values (lights and shadows) and the shapes. As soon as one starts thinking things like "eye" or "nose" one immediately start to draw a symbol - a mental image or memory - of the same thing. This takes away from the likeness in the current painting and makes the final result look sloppy and cartoonish (in a bad way).
I decided to stop at a black and white stage - and keep it fairly loose and sketch-like. I feel these kinds of paintings work well when painting on the iPad. For more detailed work I like to move over to my Wacom Cintiq at this stage, for added pressure sensitivity and feel. But up to this point I feel the iPad works just fine.