After choosing one which I felt would allow me to play around with some bold color/contrast choices, I moved on to a grayscale sketch, checking my image flow and composition now and then to be sure I wouldn't have to rework something crucial.
There's nothing too magical about the rendering phase. I work in layers, back-to-front, trying to get the 'boring' stuff out of the way early. Halfway through this part is where the doubts come in. Since the fun part, the figuring out, is already done, this is where I tend to drop an image and start another. Since I'd been doing some thinking on that lately, though, I was able to catch myself in the act and force myself to see it through (huzzah for introspection!).
Lately my main exercise is to force myself to stay loose, to keep some painterliness in the final. I failed that on the hair, where everything is rendered a bit tighter than I'd like (Poor choice of a rough brush at the midpoint required too much cleanup work), however the elbow and forearm came out, I think, quite well. Small victories :)