Step 7: Initial incision
Make an incision along the mouse's spine from the shoulders to the hips. You'll be able to see the difference between skin and muscle tissue pretty clearly, and the skin will be very loosely attached. Make sure you've cut all the way through the skin so you have a nice space to work with.
Dump a pinch of the Borax/cornmeal mixture into the incision, and we'll move to the next step.