Then, I paper-mached directly onto and over that first fabric layer. What you end up with is a hard, lightweight and textured clump that you can still cut up or add on to adjust the shape. You don't even need the fabric underneath for stability (Nightmare's shoulder, mutated arm and the majority of Soul Edge are just paper and glue alone) but I just wanted something a bit softer as a base since it would be pressed up against my actual back. In this case, I used a dark blue crepe paper to give a nice "under-painting"-like depth so it didn't turn out flat and cheesy-looking when I painted it.
As far as the skeletal portion goes, I had the unfair advantage of working at a weird shop that sells all bits of surplus items that were designed for other purposes: thus the spine is actually made out of many tiny resin kneecaps that were originally meant to be installed into miniature skeleton models...we think. (The really nice thing in this case was that the little bones were already jointed, so my tail even swings realistically.) If I hadn't come across this totally random perfect find, I suppose I would have had to locate some plastic Halloween bone decorations to modify or even cast a completely original spine. But this saved me some time, and the mini-kneecaps were overlapped spine-like and attached to each other via epoxy to the now-dried paper-covered back. Once they were on, I used hot glue as a texture over everything, adding extra muscles and veins, and then hand-painted it all.