I wanted a cast of my face: a 3D positive, as closely resembling my real face as possible. I considered using 123D Catch to model my face and then print it out on our fancy 3D printers. But that's needlessly expensive and inaccessible (except for 123D Catch, which you should look up if you haven't heard of it). This method is messy and fun, and can be done in your home.

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

You will need:

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.

Vaseline
water
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)
old newspapers
a shirt you don't mind ruining (to wear while getting your face cast)
straws
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.

Step 2: Prepare Workspace

Lay out newspapers all over the floor. A sacrificial pillow is nice, though not necessary. Cover that with newspaper too.
Pre-cut some strips of plastercloth.
Put some water in bowls so that you'll be prepared when it's plastering time.

Step 3: Cut Cardboard Splash Guard

This is the real technique innovation: a cardboard splash guard. This way, the cast-ee doesn't get too messy. This also helps to keep the alginate on the face when you're applying it.

Basically, cut a hole in the shape of your face. This will define the edge of your mask. Start by cutting your hole too small, then work it out bigger and bigger until you fit in snugly and are satisfied.

I highly recommend leaving a cardboard bib to cover your body.

In the pictures, I struggle to size my face guard. Kiteman demonstrates proper guard-wearing technique.

Step 4: Prepare Subject

Have your castee put on the sacrificial shirt, and place the splash guard firmly over their face.
Have them lie down on the prepared newspaper surface. Make sure they're comfortable; they'll be there a while.
Put Vaseline on any hairy bits: eyebrows, eyelashes, beard if that's an issue for you.
Also put Vaseline around the edges of the cardboard, to seal it.
Put straws in their mouth (and possibly their nostrils, as demonstrated by Conker-X). The straws are for breathing, so make sure they will be comfortable breathing ONLY through the straws.

Step 5: Mix & Apply Alginate

Put some water in your mixing vessel. Add alginate powder. Mix it up with your hand.
You should mix it to the approximate thickness of porridge. Or, that's what Kiteman and Conker-X said. I'm American, so I went for a Malt-O-Meal type of consistency, or a little thinner.
Try pouring some on your subject's face. If it drips right off, it's too thin. If it globs up, it's too thick.
Carefully pour and glop the mixture onto the subject's face. Advise the subject to be calm and breathe deeply through the straws. There is a bit of a panic response when your nose and mouth are both covered, and it can be instinctive to inhale through your nose. Don't do that! Be calm. DO NOT ACCIDENTALLY POUR ALGINATE INTO THE STRAWS.

Since the alginate is a bit runny, it will tend to gloop down off of the face, particularly the nose. Just push it right back onto the face where it needs to go.
It sets pretty fast.

Step 6: Apply Plaster

You don't have to wait for the alginate to dry to move on to the next step: plastercloth.

This part is really easy. Just take your cut strips of plaster, dip them in the bowls of water, and apply them to the face.

The point of this is to provide a rigid form for the alginate once it comes off of your face. My plaster-face was pretty thick; from the inside, it felt like I had about an inch of stuff on my face. This was probably more plaster than was strictly necessary.

Step 7: Let It Harden

Chill out, give it 20-30 minutes. Take a nap. Or don't; it might be scary to wake up inside that thing, and you might forget you're supposed to breathe through straws.

Step 8: Alternate Methods 1: Squash Your Face

While we're letting it sit, here is another way to make a face mold. Maybe. We didn't actually go through with this, but here it is, just as an idea.

We used Conker-X as a guinea pig, before we figured out the splash guard and how alginate should be used. So unfortunately, his alginate mold didn't turn out too well. And we were out of plastercloth. But we already had a plaster in the shape of his face, so we thought of a couple of ways to use it:

Stick breathing straws through the plaster
Carefully fill plaster with alginate, avoiding straws
Put straws in mouth
Squish face down into alginate
Hold it until it hardens

It's just a theory, but there you have it.

Step 9: Alternate Methods 2: Pour-down

This is another untested idea (see previous step) but I thought it would be pretty cool.

Put splash guard on subject
Put plaster mask on subject
Duct tape effusively (subject can take a break during this step), leaving a hole at the top
Put contraption on face
Pour alginate into hole at top (subject may need to leave some space between face and plaster)

Again, untested idea, and maybe more panic-inducing than the one I did. May invoke a feeling of drowning.

Step 10: Remove Face

Once your mask feels pretty dry, you can lift it off of your face. Lift from the cardboard, but support the alginate face; it's liable to flop out.

Assuming the plaster is reasonably firm, you can flip the piece over and let the plaster support the alginate.

Step 11: Set Up Face for Plastering

The cardboard should peel away from the plaster and alginate.

Carefully set up your mold somewhere contained and supportive, e.g. on crumpled paper in a box. It should be as bowl-like as possible in order to hold the plaster.

Step 12: Mix & Pour Plaster

Mix up some plaster the same way you mixed up the alginate. If there are directions that come with, follow them. Otherwise, I mixed mine to a nice creamy texture. I used a full yogurt-container's worth of plaster to fill my face.

Step 13: Let It Sit

Put it somewhere out of the way to let it harden. I don't know how long it takes, but I didn't come back for a couple of days.

Step 14: De-mold and Touch Up

When you're sure the plaster is hard, carefully peel away the mold.
Look, a face emerges!
There are probably a few weird bumps. For touch-ups, a file and some sharp chisels would be nice. Barring that, I used a flathead screwdriver, a rough file, and a small hex key to get into the cracks. Worked okay.