I recycled the first pair of socks I ever knit into a sweater for an American Girl (18”) doll when the pair finally wore out. The two cuffs form the front and back; the insteps and gusset make the arms. Toes and heels are discarded.

More detail: This takes both socks of a pair, and doesn’t use the heels or toes, which is generally what wears out first. Cut off the two sock tops and split each up the back, sewing the cut edges together as side seams, to form the front and back. Leave the upper part of the side seams open to sew in the sleeves. Cut the foot off off at the toes and as far up on the instep as you can; these will be the sleeves (you may have to remove some surplus from the bottom of the foot and from the gusset end, I don’t remember how much but mine does have an underarm seam). Hem the cuffs and turn the neckline in to make a wide boat neck opening; you’ll need it to get it over the doll’s head.