Put on the jeans and mark the desired ankle width with pins or chalk
Turn the jeans inside out
Remove the inner or outer leg seam stitching from the ankle up to the taper point
Fold the fabric inward evenly from the seam line to the new width
Pin the folded seam in place
Try on the jeans again to check the fit and adjust if needed
Sew a new seam along the pinned line using a sewing machine or strong hand stitch
Trim excess fabric, leaving a small seam allowance
Finish the raw edge with a zigzag stitch, serger, or fray check
Press the seam flat with an iron
Repeat on the other leg, matching both sides
Hem the ankle if the length changed during tapering