I thought I would put together this instructable on creating a creature from scratch.
It involves character concept, sculpting, making negative mould, casting, hair punching, painting and makeup therefore I hope it will help with your own stages of mask making. This one is a half face mask.
It a very low cost rubber latex "beast" prosthetic. Please remember to do all the usual tests with all the materials to check for allergies prior to application. The whole application process takes about an hour when you have produced and coloured your prosthetic after a couple of attempts and you can re-use your negative mould multiple times but if the alginate (soft moulding product poured over the clay sculpt then strenghtened with plater of paris bandages ) is left for around a week then it will become moulded and dry out as it is seaweed based.
Normal ratio is Plaster of paris 2 parts to 1 part water. Simple plastic cups are the best way to measure amounts required.The most important part of any prosthetic is to ensure you have thin edges for effective blending to the skin. This example is a little rough and needed more blending but it was done just to show the process and less emphasis was put on realism but it's still fairly effective and hope it gives you the basic idea.