Create a pattern to get the exact shape you want. Draw a stocking shape on brown craft paper or a paper bag then cut it out.
Turn the sweater inside-out and lay it flat on the table with the bottom edge lined up straight. Center your paper stocking pattern on the sweater with the foot facing toward the neck of the sweater. Lower the pattern enough so that the bottom of the sweater can be folded over to form the cuff of your stocking. Trace the pattern using a fabric marker.