Measure the distance between the bottom of the curtain and the floor. Measure the width of the curtain.
Cut out a piece of fabric to measure the height and width you previously measured, adding on 1/2 inch to the width.
Place the curtain onto a flat surface, then pin the additional fabric along the bottom edge using straight pins. Leave a 1/2-inch seam allowance as you pin.
Sew along the pinned edge using a sewing machine, then remove the straight pins. Cut off any excess thread using scissors.
Lay the curtain right side up onto an ironing board, then iron the curtain flat before hanging it back up.