There was a very helpful display about how archaeologists reconstruct the face from a skull. They make a cast of the skull and then build up the muscle tissue around it in layers. The first middle drawing shows the first stage of reconstruction.
I then worked on drawing a human skeleton, horse skull and plain's zebra skeleton in pencil then drew how I imagined their external features (I've drawn a lot of horses and zebras in the past so that helped). It would've been better if I'd taken along some tracing paper to do the different layers.