Find some 'natural' wool - brown, grey, white, and cast on about 20 stitches, join, and begin knitting in the round. You should have a tube that is about an inch and a half across, give or take.
Every few rows, decrease a couple of stitches. I can't be more specific, because it's a matter of 'eyeballing it' to get a proper mouse shape, from the base of his fat rear end to his little pointy nose.
You should wind up with a mouse that is around 2" long that comes to a point in the front. Cut yarn, and stitch a few times to secure. Make French Knot eyeballs on either side of his head with contrasting color - black is nice, although pink on a white mouse is rather splendid too.
Front being done, go to the back - and stuff the body of the mouse (from the rear) with stuffing of some kind. I use wool scraps, since I don't believe in catnip. Stitch up back end of mouse, making sure that the original tail of yarn is in the MIDDLE, so that it resembles a tail! Better yet, add two more strands and braid a nice tail.
You can crochet a long chain and attach it to the tail - I made one of these mice for my father-in-law, so he can go fishing for cats from his chair in the living room.
Mr. Tabitha loves his mouse, and I hope your cat will too!