Mount hook, tie in a thin leather or chamois strip at the bend for the tail. About one and a half shank lengths long and narrow, like a real mouse tail.
Begin spinning deer hair from the bend toward the eye. Use natural-colored hair. Spin a clump, pack it tight with a hair packer or your fingers, advance thread, spin another clump. Each clump should be packed firmly against the last. Density determines how well the fly floats and holds its shape.
Continue spinning and packing until you reach the eye. Use 8-12 clumps depending on hook size.
Now trim: the bottom should be flat so the fly rides upright on the water. The top and sides should be rounded to a mouse shape, slightly pointed at the nose. Leave some untrimmed hair at the rear for a natural look.
Optional: add small leather or foam ears at the head. Add a mono weed guard if fishing near structure.
Whip finish at the eye. Seal the head with cement.