To start, I’ll say, as a novice at prosthetics, my approach was needlessly complicated and a bit wasteful, but it yielded a product I could use, so I suppose that counts for something.  Here is a streamlined account of how I made the fins:

- using plaster strips, I made a cast of my lower jaw

- I poured wet plaster into the cast, let it set

- using oil-based modeling clay I sculpted fins onto the plaster replica of my lower jaw (covered in foil to keep the clay from sticking)

- using silicone, I made a cast of the fin as it was on the plaster face

- I painted liquid latex and bits of tissue (for structural support) into the silicone mold, about seven layers worth

- after peeling the finished latex fin from the mold, I trimmed it up and painted it grey with three layers my own blend of PAX, the same blend I use for my ears, neck and hands (being careful to set with neutral powder between each layer).

This tutorial should help give a better idea overall of what process I was going for… but really, there was a lot of this I could have skipped, I just had no idea what I was doing.  I hope my trial and error helps someone do it better!

As always, if there are more questions, drop me an ask!

EDIT:  I had a good question about application.  As for applying the fins, both PAX and spirit gum have given me trouble, but so far the most successful method has been:

- coat the backs of the fins in spirit gum
- place them as desired
- peel them back up immediately and let the gum get tacky (about 30 seconds)
- place the fins neatly back in place on the cheeks, and press firmly

Much the same way one would use rubber cement or other contact cement, I suppose!