You would only need the front half of your face cast - a bit past the starting of the ears for this and most purposes. A complete head cast is more work, material and weight. For this you could manage with simply casting from the forehead to a bit past the tip of the nose, a partial face should be enough. I've done this numerous times for producing prosthetic pieces.
You will need assistance, life casting your own face alone is not impossible but fairly close to it. I trained two assistants to help me with life casting and then had them apply the mold to me. I applied alginate in the critical areas around the eyes and tip of the nose while they did most of the rest and did the plaster bandage layer. After this was removed, I took over the casting process to prevent problems.
I would advise practicing by doing small molds of just your nose or chin (or someone else's) to get the hang of the process while using less materials. It's easy to capture air pockets and make various other mistakes in the molding and casting process, so you might want to start small to get the hang of it.
What you are creating is in between a prosthetic and a traditional mask. This is sculpted in either oil base or water-based clay on top of your life cast to create a precise fit. if the final product will be silicone, use a sulfur-free clay . A separator might or might not be applied depending on the materials used. A mold wall is created around the perimeter of your sculpture, then rigid mold making material applied.
Your mask(s) will be cast within this mold. Molds can be made of plaster, hydrocal, Ultracal, dental stone, various rigid urethanes or fiberglass depending on your budget, how long do you want the mold to last throughout repeated castings, and the material being cast within it. Plaster is the least accurate, hydrocal is a bit better, Ultracal is much, much more accurate but really was adopted to be used with foam latex where you put a tremendous amount of pressure on both parts of a two-piece mold and then bake it which would destroy regular plasters.
Natural liquid latex (polyisoprene) is traditionally cast in a plaster mold so that it can absorb the water and ammonia out of it, although it can be brushed in layers into molds made of other materials. If you're going to use a urethane, silicone rubber, thermoplastic or other similar product for the mask you would fill your negative with the material and then force your life cast down within it. A two-piece mold like this provides an interior negative surface for the finished piece that fits against your face precisely. This is the type of thing that you have to do with prosthetics, while Halloween masks are simply hollow rubber bags that approximately fit over most people's faces.
This is closer to a prosthetic but it doesn't need to be quite as flexible as one that is passing for regular skin. Take a look at the videos from a manufacturer like Smooth-on to learn more.
