Sorry the picture of this isn't very good.
I used worsted weight Caron Simply Soft in dark country blue and an H hook. Gauge is not important in this pattern.
Chain 52 (make it longer if desired)
Row 1: Hdc in third chain from hook and in each stitch across. Chain 2, turn.
Row 2: Hdc in first stitch and in each stitch across. Chain 2, turn.
Repeat row 2 until you have the desired height. (I did six rows.) Do not finish off.
Row 1: Chain 1, work round of sc all the way around, making sure to work 3 sc in each corner. Join with slip stitch to first sc.
Row 2: Chain 1, work one round of reverse single crochet in each stitch. Join with slip stitch to first reverse sc. Finish off and weave in ends.
Using a safety pin, wrap the tieback around the curtain as tight as you want and pin in place. Do not pin it to the curtain.