Pick a nice radish with oval shape. Trim off the green top leaving a small amount of stem (nose). Cut off a thin slice of the radish to let the mouse sit on the surface (with the tail up).
Cut slots for the ears on each side. The slot should be slightly thinner than the ear itself. For the ears you can split the part you cut off earlier into two parts, or better use two slices of another (slightly smaller) radish.
Slip the ears into the slots and you are done. For the eyes you can use cloves or just peel away a small part of the skin with a tip of a sharp knife.