cool, first time I've seen someone try to take on skull face. If you plan on doing the full head, it would probably better to mold it as one piece, (just follow step 2/3 but for the entire head) however, if you are just looking to make the mask, the process is about the same, just less costly and hence labor intensive. as I see, you have three main options:
1. make it out of leather, or a leather like material. patterns for this exist, and can probably be found if you searched for "Incredibles mask pattern" although i cannot point you as to where to search. (also note that pleather such as Naugahyde can be heat molded, helping to hold a more dynamic shape)
2/3. for this/these methods the processes are initially the same as each other, only the final construction differs. when you heard clay, they were not meaning put the clay on your face, but rather sculpt the piece out of clay and then mold it. to do this, either make a life cast of your head (or Skullface's head if your using a pre-built full head mask) or buy either a Mardigras plastic mask that looks like a human face or even a foam hat makers head/ hat display stand (last one not recommended, unless you mold with latex, but more on that later). on top of this, simply use clay (oil based seems to work the best) and sculpt the piece on top. when done, the head you chose will be wearing a solid clay mask. make this as smooth and perfect as possible, as it will be more difficult to clean up later on. at this point, most spray this with clear spray paint, but this is not a necessity. paint with one thin coat of "Plaster of Paris" roughly the consistency of fully melted ice cream. When dry, pour more plaster on, at roughly the consistency of applesauce, so that the mold is about 2 cm thick. LET THIS DRY THOROUGHLY!. remove the original sculpt from inside the plaster mold, leaving s perfect impression of the mask. clean the mold, and wax the inside, to act as mold release. if you plan on using rondo (explained soon) silicone spray also works. finally, make the mask by painting the mold with in either liquid latex (pre-dyed see package for details on this), or "rondo" ( 50/50 mixture of fiberglass resin and bondo body filler). repeat this process until desired thickness has been attained. if you are using rondo, you can even include some fiberglass for added strength. wait for the mask to cure COMPLETELY, and then remove. finish the piece as desired, but only paint latex with latex paint, and avoid doing even that, if possible. with latex, you will have a flexible but limited use mask, while rondo yields a very tough rigid end product (meaning it is entirely inflexible). for more information, watch this video.
https://www.youtube.com/watch?v=M9yyf-IEk_k
process 1 may require some degree of sewing skill, but will only run you about 30 usd
process 2/3 requires more time, money and effort but the mold can be used multiple times, and you may end with a better looking final product, but will probably run closer to 80 usd.
best of luck, if you have any more questions, i'l do my best to answer!
RNC