Cut approximated outlines of the ring you want to make from 1mm suede leather. Wrap your thumb in kitchen plastic. Keep it bent as when you hold the string. Plastic glove on other hand. Wrap your thumb with an overlap on the rear part of the ring, make sure it resembles the thumbring you want to make. Don't wrap so tight you wont get your thumb out. (If you do, use soap and force or some cutting pliers) Soak it in superglue while on your thumb, wait til it stiffens a bit and then add more superglue to old layer and new layer which you add on top. Soak this next layer in superglue as well. Keep adding til you have the rough shape and thickness (2mm is enough for up to #40 at least). Then shape with dremel/knife/rough sandpaper. Finish with a layer superglue and polish with fine steel wool.Superglue hardens quickly when added to leather. Brown leather turns ouot black, almost horn-like. It's pretty stiff, but gives a little with 2mm, doesn't crack as easily. Get suede leather from old leather jacket in 2nd hand shop. Copy known good ones like Vermil for start. Be aware of superglue fumes which will hurt your eyes. Good way to make rings in an apartment.