Such fun to see a loud burble of water that this mouse causes when that take it in. They like it so much that it is often deep in the throat. Make sure you have some long nose pliers or hemostats.
A very east pattern to tie using thin strips of mink. You don't need expensive materials. I've been using scraps from a mink coat maker. The tail is the trick. It can be a scraggy piece with just a wisp of hair at the end.
Get the tail in. Thin strip of foam tied on. Wind a thin strip of mink. Then bring the foam over. Tie down, come back a bit and tie down again. Shape the "ears". Colour the foam as required. The ears make it easy to see where the fly is and creates a bit of noise moving the water when jerked. The hairs move all over the place driving the bass mad.