Alginate is a miracle substance used by dentists to take moulds of teeth. Also used by sculptors to create casts. The easiest cast to do is your hand, mix up the powder with water in a container like a plastic bottle. Keep your hand in there until the alginate sets, and then very carefully move your fingers until they become loose and you can remove your hand.
Then fill the alginate mould with plaster, tipping it round the mould, ensuring there are no bubbles and the plaster reaches the fingertips. When using plaster be aware that it heats up as it sets, never attempt the make plaster casts directly from your skin as you could get serious burns. When the plaster is set, pull away the alginate bit by bit. You will find the alginate has picked up every detail of your skin, including finger prints.
Making a cast of the head is essentially the same process but trickier and requiring at least one other person. Some people find the experience claustrophobic and you must be willing to remove all alginate immediately. Talking to your model throughout and explaining each stage is essential for keeping them calm. The first stage is to tie hair back, remove make up and apply vaseline to eye lashes and eyebrows; this will allow the alginate to come away easily. Your model will need straws in their nostrils to ensure they can breathe. The alginate must be applied quickly after being mixed before it begins to set. It will feel cold and lumpy. After this apply strips of mudrock (plaster soaked fabric) over the alginate to give the mould support. Removing the mask is the trickiest stage as the alginate is delicate and easily torn.
Any holes should be filled with clay so plaster does not leak out, and then fill the mould with plaster. Seeing the final mask is uncanny - the nearest to a lifeless face you will come. Death masks are also made in this way. Whilst every wrinkle and pore are replicated, the life of the sitter is completely absent. The result is a shock of mortality.