i'm not much of a mask maker i just know a few basics. i would advise hanging out in the Sculpture and Makeup Effects section of this site.
peruse the many threads on the subject there. they would know far more than i would.
i got a gallon can of Mold-It somebody gave to me i experiment with from time to time. it's liquid latex rubber. says on the can to let it sit in the mold for a time or slosh it around in there untill a thickness builds up then pour the unused excess out back in your can. it suggests a coating of shellac or spray sealant on the pourous inside of the plaster mold. it also says to bake in the oven real low no more than 130 degrees f until the rubber cures. dust the inside with talc when done so the insides don't stick together when you pull it out the mold. them's the basics but i would get several more opinions on the matter.
what i described was just the quick, cheap, dirty oldschool way.
...and in no way was my advice complete.
good luck and get thee to the other parts of the forum.
don't burn the house down!