First, I had to look it up to see what a Dalek head was. :-)

Would it be possible to link to a photo of what you've made so far?  Is it just the dome on the very top, or does it include the rings that go around it and other attachments?  I hope it's just the dome, and you're attaching the rings afterward, or we shall be getting into the realms of a very complicated mold, far beyond what I've ever attempted.

If it's just the dome, it should be relatively simple.  First, the positive needs to be at least 3 days old to be cured.  It would be best to do this on a piece of plywood so you can move it off the table without damaging the mold. Cover the plywood or tabletop with a large piece of plastic (preferably the heavier type- you can reuse it).  Set the dome on the plastic.  If you have some non-drying Plasticine clay (kid's play clay), roll out a rope of it and make a circle of it around the positive (not too close) and press the clay onto the table plastic, forming a wall or dam to confine the wet plaster.

Then either apply an oil  cooking spray like Pam and wipe off any excess.  Or, for a sure-fire release you could spray the Pam (don't wipe excess) and then apply a layer of plastic food wrap (cling film) carefully over the Pam and press it down (start at the center and work toward the edge), creating as few wrinkles as possible (the Law of Wrinkles says you're going to have them, no matter what you do).  The Pam will hold the cling film in place.  Tuck the ends of the cling film under the positive dome.

Mix up your plaster as directed, and slowly pour or spoon it over the dome.  Using a spoon or putty knife, keep scooping up the liquid plaster that flows off the dome and reapply it to the dome, aiming for a final thickness of at least 2.5 cm (1").  Depending on the type of plaster, it usually gets thicker fairly quickly, so you will be spreading/pressing it on instead of just spooning the liquid.  When you get it about right, run the back of the bowl of the spoon over it to smooth the whole exterior surface, and then let the whole thing set, undisturbed.  You must do this all without interruption.

You are probably aware that plaster does not DRY, it CURES by chemical reaction.  First, it hardens.  Even though it feels hard, it is quite fragile at this point, so leave it alone.  If you have to move it, move it on the plywood.  After a few hours or a day, you can carefully lift the new mold off the positive, and set it in a safe place to continue curing, or you can just leave it where it is.

It will take three days to cure, and there is no way to increase the speed; don't bother trying to put it in the oven or aiming a fan at it.  It just takes three days.

Once cured, separate the new mold from the positive if you haven't already, and set it face-up.  If it tries to roll or shift, block the sides with thick sponges or small blocks of wood.

There are a couple of schools of thought on how to treat the mold before you start to apply the papier-mâché. 

1. You can brush or spray the inside of the mold (including the edges) with shellac or varnish (blot up any puddles), and then spray with Pam or other release.  Personally, if you're using papier-mâché pulp, I would do this. But don't use this method on a borrowed ceramics mold or the owner may inflict severe physical damage, as it does ruin the mold for future ceramics use.
2. You can spray the plaster directly with Pam and go straight to applying the papier-mâché.
3. You can leave the plaster "raw", and apply moistened paper without glue directly to the untreated plaster and press it into position, using two layers.  Then start applying the paper moistened with glue.  That's MOISTENED, not sopping wet.  If it's too wet with glue, the glue will soak through the unglued original layers, causing them to stick to the mold, which will cause trouble when you try to unmold.
4. Some people use a) a couple layers of dry paper directly against the raw plaster, or b) apply it wet and let it dry before starting to apply the glued paper.  I've never tried these last two methods due to my naturally impatient nature.

Once the exposed papier-mâché seems fairly hard, remove it from the mold and continue to let it dry, as the side lying against the mold will still be moist.  You can speed the drying by setting the piece in front of a fan or in the sun, but excess heat from an oven tends to cause warping.

Be sure not to drill or paint (etc) until the papier-mâché is completely dry.

Sue

You're welcome.

Yes, I've seen the instructions to use those methods.  The plaster-embedded bandage material is somewhat expensive, $45 for a box of 12 rolls 25 yrs ago.

Using hessian is okay, but when I tried it, it was so time-consuming to get it smooth that the plaster hardened faster than I could work.

And I forgot to mention that plaster BURNS skin.  It gets that hot when it starts curing.  So don't use your hands to handle the moist plaster.

While you can use a couple layers of the plaster/bandage material to make molds of body parts, NEVER, EVER use liquid plaster for that purpose, and this is why:  http://www.mirror.co.uk/news/uk-news/ar … ght-424409

And don't pour any residue down the drain, either, as it hardens under water and blocks the pipe.

Good luck with your project.

Sue