(I have to say I make a very fine Barristan Selmy)
After I shoot a bunch of photos with my camera I load them onto my computer and start looking for the photos that match the look and feel I had in my rough sketch (also always be on the lookout for something different and interesting in the photos that you did not think of when drawing the sketch, the more photos and different poses you use the better chance of finding the right image or series of images.
Once I have found an image that I like I bring it into photoshop. I then lower the opacity on it and start tracing over the image onto a new layer. Lowering the opacity makes it easier to see both the drawing and the important information that I need to see while tracing, you dont want the image too dark or saturated or you wont be able to find any of the important lines or edges.
I make a quick initial pass of the photo only tracing the important landmarks like the face and hands and any special props that I have in the image. After that is done I turn off the layer with the photo and I start refining the drawing, making the arms thicker, adding more of the drapery and fabric of the costume, changing the pose to get it to fit and feel the way that I want it. Once the drawing is finished I then copy and paste it into the document with my final drawing, there I will size it, rotate it, and maybe even make more changes until it sits and feels right in its placement and context of the overall drawing and composition. I then repeat this step ad nauseam for every figure (in the end there should be at least over 20 figures!)