Then you cut a heart shape out of newspaper; lay it on top of the newly washed/shrunk sweater with the pretty round buttons buttoned up. Cut around the pattern, leaving about 1" for seams, and after turning right sides of the sweater together, stitch it up on the machine (or by hand).
Also: hand-stitch down the opening of the front with the buttons in place, so that opening will be closed and you won't have gaps. Turn the heart right-side-out and stuff it as fat or as flat as you like with poly stuffing or bits of left-over quilt batting, etc. and then hand-stitch closed the little opening you left in the side seam for stuffing.
These can also be made into smaller heart shapes and filled with dried lavender for sachets. Or pin cushions. Or tree ornaments. Let your imagination go wild! Also: it doesn't have to be a sweater; anything made from 100% wool will shrink and "felt".......like vintage skirts or coats or men's slacks/vests/sweaters. Now, go make some "vintage" Valentine gifts for special people in your life!