Weave in all ends. Pull out provisional co and place stitches on spare needle. If you plan to add an elastic into the casing, get your elastic and measure it out so that it is snug but not too tight, about 2”/5cm below your natural waist. Sew the elastic ends together and position in place.
Align the first st of the co with the first st of the live round; knit those two stitches together as a single stitch. Repeat this all the way around the waistband. When you are done, you should still have 126 [142, 158, 174, 190, 206, 222] sts, and your casing will be closed and folded along the purled picot edge.
The purl picot round is now considered the first round of the garment. This means your garment currently has 11 rounds total.