Step 9: Plaster the other half
Tidy up the edge of the plaster mould by snapping or sawing off any unwanted blobs of plaster. Once you've done this, cover the exposed plaster with tape so that it doesn't stick to the second batch of plaster.
Now repeat the previous step to cover the second half of your Alien in plaster.
Once this has set and cooled, carefully separate the two halves of your plaster mould and scrape out all of the plasticine. You should be left with a two-part fossilised negative of your Alien.
Don't worry too much if you've managed to crack the mould by mistake, as you can usually repair it with glue and some carefully applied plaster.