The molding process goes very quickly, so you'll want to have everything, including the model, ready. In my case this included instructions to the young models to place their hands into the molding compound open, with fingers spread apart, wiggle their fingers and twist their hand a little, then closing their hand (or holding whatever position they chose). I had them go through the motions a few times with their eyes closed - they rely a lot on hand eye coordination, and they won't be able to see their hand inside the molding compound.
I did not instruct my children to wash their hands before molding. The natural oils act as an ok mold release compound, although clean hands should make a better impression of fingerprints and tiny features. They should certainly be instructed to wash their hands afterwards.