Curtains that are too long will not hang properly. If you are certain the curtains will not be moved to another window, shortening them is the best approach. This means removing the excess fabric, so the curtains will not be usable on a longer window. Because rod placement varies from window to window, you will get the best fit by hanging the curtains over the window, on the rod you will be using when the project is finished, and marking the spot on the fabric where you want to hem to fall.
Measure the curtains with the yardstick, from the top edge to the desired hemline, and add 2 inches.
Spread the curtains flat on the work table, and mark the length you just calculated at each end and in the curtain's centre with the tailor's chalk.
Place the yardstick across the chalk marks, and connect the marks, making a straight chalk line.
Cut the excess fabric off, using the scissors and following the chalk line.
Place the curtains face down on the ironing board, one at a time, and turn the raw edge up and over 1/4 inch. Press the fold in place with a hot iron.
Make a second fold, this time 1 3/4 inch wide, and press with a hot iron.
Place a straight pin every 6 inches to hold the folded fabric in place, and sew 1/8 inch from the folded edge, using a sewing machine set to a medium-length straight stitch. It will look as if you are topstitching the folded hemline when you are looking at the back side of the curtain panels.
It is possible to get the right length by measuring the window, but the rod's placement can throw curtain length off by several inches, depending upon how high the rod is placed over the window.