The hardest part of this ornament is cracking the walnuts correctly.
You will need:
Whole walnuts (and a nutcracker)
Hot glue or Krazy Glue
Carefully crack the walnuts along the line; the goal is to get a perfect walnut shell half. Empty out the walnut meat and eat as desired. The pointier end of the walnut shell is the nose of the mouse. Use the marker to put a nose on the little beastie. Draw some whiskers or make them out of pipe cleaners, if you desire. Cut out little half-circle felt ears; glue them on. Give the mouse some googly eyes. Glue a tiny bow on the mouse's "neck." Glue a pipe cleaner tail to the underside of the other end. Take a six-inch piece of pipe cleaner and glue it to the underside of one side; you use this to attach the mouse to a branch.
(Alternately, dry out a mini bagel that has had part of its top "eaten" by putting it in the oven on low heat for several hours; spray lacquer it and glue the walnut mouse right next to the eaten section. Attach a loop to the bagel and glue a ribbon to it.)