So I got some materials online, a (mostly) willing volunteer, and some helpers, and we set about things.
I got a lot of help from this instructable, but we modified the process enough that I think it deserves its own step-by-step.
Why did I want a cast of my face? It will all be explained in time. But rest assured, I have further plans for it.
Step 1: Materials
plastercloth (it comes in rolls- get a couple of rolls)
alginate* (a 1lb jar of dermagel is plenty for casting a face or two)
plaster powder (I used something like three cups per face? I didn't really measure.)
the above things can be bought at Douglas and Sturgess online.
a mixing vessel- I used a biggish yogurt cup (see pictures later)
scissors you don't care too much about
a cardboard box or sheet of cardboard
ideally, a box-cutting knife (for cutting the cardboard)
a shirt you don't mind ruining (to wear while getting your face cast)
a couple of bowls for water
*A note on the alginate: I've plaster cast my face and those of others before, but I always just used plastercloth with some Vaseline on the eyebrows. So why use alginate? Two good reasons. Firstly, the internet now warns me that people have lost bits of themselves to plaster molding themselves- plaster heats as it sets, and can get quite warm. It can burn you right to pieces, apparently. I think this is more of an issue if you're pouring mounds of plaster on something that if you're just using plastercloth, but good to keep in mind. The second reason is more important to me: you get wonderful detail with the alginate. Plastercloth is going to make you a mask that fits nicely to your face. Alginate is going to give you a mold that really looks like you.