Cut a piece of shearling, approx. 9" long and 4" wide. Because of the shearling's thickness, it must be trimmed down to fit a can or bottle. I trim the shearling to about half thickness.
Dry fit the shearling. That is, place the trimmed shearling in to the koozie body, with out contact cement, to check for placement as far as how far down in the body it goes, and how tall it is. Trim to fit.
At this time you can also try installing a bottle or can. It should be tight for either. If too tight, meaning the can or bottle won't go in, trim some more shearling off.
Once you have the shearling cut to fit, put some contact cement on the grain side of the shearling. about a 1" strip, near the top. Now put a 1" strip of contact cement near the top of the koozie body, near the top, on the inside.
Roll the shearling piece, with contact cement applied, so that the contact cement comes in touch with nothing. Insert in to the koozie body. As you insert, allow the shearling piece to unroll. Work quickly and move the shearling piece into place allowing the two contact cemented areas, the koozie body and the shearling, to come together. You, at this time, have to work the shearling down in the koozie body AND where the two ends of the shearling will complete the circle inside.
Picture of a top view and side view of a completed koozie, showing shearling will follow.