Whatever you're using for a head form, grease it up with with vaseline.
Lay down some wet plaster tape in a rough shape of the base head cap. I did about 3 layers, and let it harden fully before removing it and trimming it to the right shape.
Make the horns. For things like this, I like to start with a base of crunched up aluminum foil - it's lightweight, easy to form, and holds shape well. I took the time to make two symmetrical horns.
Using thin strips of wet plaster tape, wrap and smooth a couple layers of plaster from the tip down towards the base of each horn.
With the cap part centered on the head casting, use more strips of plaster tape to securely affix the horns to the cap base, taking care to keep them symmetrical. Then, let that harden completely.
Using some "Fast Mache" paperclay, fill out and smooth over the horns. Let that dry completely.
Spray the horns with Super 77 spray adhesive, and wrap them with twine. Let that dry completely.
Once the whole thing had dried *completely*, coat it all with a layer of black Plasti-Dip. This will protect the plaster from outside moisture. Once that is dry, paint it with black PAX paint, for a nicer finish.
Drape a small piece of textured black fabric (stretchy) over the headpiece, trimming it and cutting holes out for the horns.
Once you're was satisfied with how it looks, spray the back of it with Super 77, and glue it down to the headpiece, smoothing out all the wrinkles, etc.
Drape a piece of shiny metallic black lycra over it, trimming to form the "V" trim. Once satisfied with the shape and fit, glue it into place with Super 77, folding edges to the underside of the cap.
Using a larger piece of black metallic lycra, drape one side of the ... I don't know what I'd call it. Skull cap? Cowl? Sort of? Anyway, drape that on one side, pinning in place.
Use another large piece of black metallic lycra to drape the other side, overlapping the first.
Once you're happy with it, hand stitch the pieces together, sewing right up to the horns to secure everything in place.