How you mold it depends on what you want the final product to be. The rule of thumb is flexible cast/ rigid mold, and rigid cast/flexible mold.
You say a "Mask" sculpt? Would I be correct in guessing that you will cast latex into this mold? That is a 'flexible' material, so a rigid mold made of gypsum would be the way to go. If I am wrong here, please clarify.
The critical thing with any mold of any type is the parting line, to divide the two halves of the mold for casting. In this case, the reasonable line would be along the sides of the neck, along the outside of the ears, and across the top of the head, thus separating the front from the back.
Here is an issue with parting lines that I can't determine by your photo: If the sculpt is soft and would deform under its own weight, it cannot lie down for forming the parting line and will have to remain vertical. If the sculpt is solid enough to lie on its back, then you can build clay up to the parting line for molding. Building a vertical wall for a parting line is more difficult but not impossible. It just takes more planning and support.
I like to use water clay for the parting lines, as it is easy to work with and it is cheap and disposable. The clay is brought up to the parting line and a nice, flat surface is smoothed out for the two halves of the mold to mate back together. How clean the clay meets the sculpture will determine how good of casts you will get out of this. the clay should make a clean, 90 degree contact with the sculpt. Any variations will produce a flashing of cast material to be trimmed.
Lastly, based on how solid the sculpt is, the clay parting flange needs to be tooled to a clean mating to the sculpt. (meaning a nice flat surface mating at 90 deg. , as mentioned above) If the sculpt is soft, it may deform as you use a tool to apply the parting flange. I couple of coats of sealer (read: Lacquer) will keep the tool from digging into the sculpt and give a clean parting line.
If you are going to do a gypsum mold for latex, it should be done in several layers: A thin, beauty layer, a thicker strength layer, and a final layer backed with fiber ( cheesecloth or hemp)
If I am wrong on the intent, then I will correct my advice. good luck on your project.