Out of felt, cut a heart shape, 3 by 3 inches, for the body and two circles, 1 inch in diameter, for the ears. Cut a piece of sturdy cotton thread 6 inches long for the tail.
Fold the heart shape in half and stitch, right sides together, catching the tail and the ears as you sew and making a small fold or pleat in the ears (as shown in the diagram). Leave 1 inch open at the back of the mouse. Turn it inside out.
Stuff the mouse with dried catnip. Sew up the hole by hand.