When you first start investigating making prosthetic appliances, most of the sites you'll come across will tell you that you need to work off a lifecast of your own face to make well fitting prosthetic appliances. While that is the best way to go about it, if you are on a tight budget, don't have someone to cast your face for you, or you are just doing a few pieces, you can use a styrofoam wig head. They are about $10 at almost any craft store. Make sure you get the male foam head, and not the female. The female heads are muuuuch smaller than anyone's actual head, and their features are somewhat smoothed out and are rather unusable. The male forms are closer in size to actual heads, and they have ears and noses, which the female forms don't have.
For sculpting, I prefer an oil based clay. There seems to be a trend in the sfx world toward water based clays, and while there are some advantages (like being easier to clean out of your finished mold, for instance), sculpts in water based clay have to be covered while you aren't working on them to keep in the moisture, you need to keep a spray bottle on hand to keep the clay from drying out, and it has a limited shelf life. Oil based clay will live on indefinitely if you keep it in a sealed container (like a ziplock bag), you can set it aside for months and not worry about it drying out, and it's completely reusable. It can be a bit of a pain to clean out of a mold, but I think the benefits outweigh the annoyances.
My favourite brand to use is called Plastilina. It's a little pricey at about $4/lb, but since it's reusable, it's worth the price.
available on Amazon right now for around $6.
As for sculpting itself...I can't give you many tips here. Don't try to rush it, especially if you don't have any prior sculpting experience. Take your time, and use lots of references and sketches to guide you. Don't forget small details like skin texture, scarring, pores, etc. They'll keep your final piece from looking too flat and smooth, and not like a real piece of skin.
Once you have your sculpt finalized, the next step is to make a mold of it. I like to use UltraCal 30 to make my molds, but it's a little hard to find if you don't have a mold supply store nearby, and can be expensive to ship, as it tends to come in large quantities, like 50 lb bags. A cheap, readily available alternative is Plaster of Paris. You can get it for super cheap at any craft store, and it does the job rather well.
To create your mold, you can either build a wall around your piece, or you can simply glop the plaster onto your sculpt. (Be sure to cover your workspace with a plastic tarp in case of spills!) Keep in mind, this is for one piece molds, and not larger, two piece molds. Two piece molds are a different animal altogether, and I'll address them in a future post.
Casting in plaster without a wall:
To build the walls, simply use some extra clay to build a box around your sculpt, leaving about an inch of space all around the sculpt. This is best for small pieces like ears, noses, and chins, but generally won't work for pieces that cover the entire forehead. Once your walls are built, simply pour the plaster into the box you built with your clay, and allow it to set. Make sure to tap the foam head a few times to work all the bubbles out of the plaster, so you don't have voids in the plaster once it has dried.
Molds made from walled sculpts:
For larger pieces, or if you just don't want to go through the trouble of building walls (I usually don't bother with walls, really), you can simply apply the plaster directly to the sculpt. Be sure to work it into all the little cracks so that it captures all the detail you've put into your sculpt. Keep building layers of plaster until you're satisfied with how thick your mold is, and then add a "beauty coat", which you can smooth out with your hand to give you a nice smooth surface to the mold.
Right now, I cast all of my prosthetic pieces in liquid latex to get my final pieces. I do this mainly because it's cheap, and it's readily available at my local costume shop.
I like to put down a thin initial layer to make sure I get nice thin edges (thin edges make appliances easier to blend with your own skin when it's time to wear them), and then build up toward the inside of the appliance. It's best to let each layer dry completely before adding another one on top of it. I like to have around 8 layers in my final piece.
Once your appliance had dried completely, powder it liberally with setting powder and remove it carefully from your mold. It helps to have a makeup brush and some setting powder on hand to work powder onto the piece as you expose surface area while you're demolding. The powder will keep the latex from sticking to itself, which it loves to do if you aren't careful.
Your first pull may be perfect, or it may be a hot mess. It takes a little bit of practice to pull a piece without it sticking to itself or otherwise mangling itself. (Such as, if it's not completely dry when you pull it, it could just turn to mush when you try to remove it from the mold. Not that I've ever been impatient and pulled a piece too soon. Nope. Never done that.) Don't worry if your first pull, or even your first few pulls, are not perfect. The mold is fully reusable, so you can make as many appliances as you need.
Good luck with your pieces, and I hope I've been a bit helpful. If anyone who reads this has questions or needs clarification of something, please feel free to comment, and I'll help in whatever way I can. Good luck!