What they said. I have a small mitre box with some pins so that you can ajust it to fit your work. A small vice, used carefully can work. Placing something softer than the metal between the vice and the work, like some plastic can help to prevent scarring the work. There's also small model vices with plastic jaws specially designed for such things. I have used exacto style hobby knives, but only on small work. I've also used diagonal cutters occasionally for smaller cuts. They'll make a bit of a mess of one side, but the other side can be fairly clean. If you're removing a limb, say, and intend to replace it and don't need the cut side of the removed limb to be especially clean right at the cut. This only works for small cuts, of course, but even heads aren't usually out of scope. (Just don't expect the chins to come through completely unscathed. Beards are great fixes.)