After a bit of searching online, I found a good video on how to draw using a grid. This is a method I use a lot in order to enlarge photos or prep sketches, such as this one, which had to go from 7 x 6 inches, to 7 x 6 feet:
This tutorial shows how the small drawing above becomes the wall you see below. The painter in the video uses a 1:1 scale, since he's creating a small portrait, but the principle is the same.
If you want to try drawing and don't know where to start, using your snapshots and a grid can help. It's a good confidence-builder because it lets you draw in small, manageable bits, instead of plunging in over your head.
I still use this method when I need super-accuracy in an animal portrait or when I need to enlarge a source image.