One year for Halloween we made our own face casting molds with plaster of Paris. This is very easy to do and a lot of fun as well.
We bought the plaster of Paris at our local hardware store and used this along with water to make the mixture for the molds.
To shape them to your face, we applied something like Vaseline all over our face so the mold would come off easily. We also put straws in each nostril so we would be able to breathe as we were making the mold to our face.
It works best if you do this on another person instead of trying to do it yourself. It took about 20 minutes for the plaster of Paris to get hard and create the mold.