Remove your shoes and socks. Slip one foot into the sleeve of the discarded sweater. Pull the sleeve cuff up your ankle until it reaches a comfortable spot on your leg for a sock edge to end.
Place your foot on the floor, encased in the sweater sleeve. Draw a line on the sleeve 1/2 inch above your toes. Use a contrasting color marker for easy visibility. Measure and mark the other sleeve with your other foot.
Remove your feet from the sleeves. Lay the sweater on a work surface with the marked line on top. Cut the sleeves along the marker lines.
Turn the sleeve pieces inside out. Sew the cut line closed, stitching 1/4 inch from the edge of the fabric. Make another stitching line just inside the first to reinforce the toe of the sock. Stitch both sleeve pieces in this same manner.
Turn the socks right-side out and slip them onto your feet.