I inserted the mouse head form just like in the Mouse Taxidermy
Instructable, and trimmed the wires so they wouldn't interfere with the rest of the project.
We inserted the (hardware) mouse into the shoulders first, then pulled the tail region around the back. It was a tight fit - next time we'll find a fatter mouse, as it's easy to fill extra space with cotton balls! Skin is stretchy, though, so long as you keep it moist the mice should integrate nicely.
Sew up the back opening, starting near the tail and working towards the head. Leave a space for the scroll wheel- it won't work properly covered in fur!
Tack the skin down around the scroll wheel with a bit of superglue, taking care not to gum up either the buttons or the scroll wheel. I usually apply the glue with a tool, either a pin or a piece of wire with a small loop on the end.