If I were going to guess, I'd guess that the dermal is a silicon rubber sheet with a bunch of tiny piezoelectric sensors stirred into it, and the fixative gel is the same rubber or slightly more flexible, plus a mild solvent that has it temporarily gooey; not at a high enough concentration to dissolve the dermal, but sufficient to fuse it to the softer rubber that the gel sets into as the solvent evaporates, and to make the dermal itself more flexible/shrinkable for a while until exposure to air evaporates the solvent. During that time it would be soft enough to shrink or expand somewhat to fit, and you could splice patches of it together although that would likely leave a visible seam. The result would be a thin "skin" of slightly harder/tougher rubber fused to "tissue" that's softer and more flexible.
So the procedure would be a matter of putting a pre-formed head covering over the fixative gel, probably bringing a seam together at the back of the head to make it reasonably snug, and working the air bubbles out from under it to make sure all of it is in contact with the gel. At which point the silicone rubber contracting over the gel would do the final shaping to a perfect fit. Most of the tools would be the same sort of thing you use applying fiberglass coverings.
The sensor net would be piezoelectric crystals - the same microscopic bits that used to get attached to phonograph needles to detect vibrations. That could provide a fairly good sense of touch - but depending on how the sensor signals are detected, probably one that would degrade over time as tiny little conducting fibers in the dermal wear out depriving the embedded microelectronics of their antennas. So May probably now has a drastically better sense of touch over her face than she got with her out-of-warranty ReformChassis.
Of course this is all me being a maker weenie and second-guessing the artist. It's plausible, but doesn't necessarily match canon.