Measure the desired finished length and mark it with pins or fabric chalk
Hang the curtain and let it settle for 24 hours before hemming if needed
Remove the curtain and lay it flat on a clean surface
Trim any uneven edges if necessary
Fold the bottom edge up to the marked hemline
Press the fold lightly with a low-heat iron if the fabric allows
Fold again to create a clean double hem
Pin the hem in place along the entire edge
Use a needle and thread for a hand-sewn hem or a sewing machine with a fine needle
Sew a straight stitch close to the inner folded edge
Keep stitches small and even
Remove pins as you sew
Trim excess thread after sewing
Rehang the curtain and check the length
Adjust and re-hem if needed