fold a semi circle of fabric in half right sides together, and sew down the straight edge. I used a machine but you could easily do it by hand, if you used small running stitches or backstitch.
turn it the right way out and stuff it with wadding, cotton wool, or even some lavender. it wants to be quite firm, and make sure you push it right to the tip - the nose.
then sew a running stitch all the way round the edge.
and pull it tight, then stitch across the hole a few times to secure it.
measure across the base of the mouse, and cut a circle of card that is about 1cm less in diameter. cut a circle of fabric slightly larger than the card circle, and then sew a running stitch around the edge. lay the card inside and pull the stitches tight.
sew to the base of the mouse, using tiny stitches just catching the fabric (not the card).
cut circle or teardrop shap ears from felt, pinch and sew them near to the nose, make some eyes with french knots, and stitch some strands of thread across the nose for whiskers. attach a tail made of a piece of wool, ribbon, or string, and your mouse is finished.
now you just need to give it a name, feed it some cheese, keep it away from the cat, frighten someone scared of mice with it, make it a friend. hope you have fun.