Summarize this article:

Your last round should reach the bottom of your base round without stretching. You may be tempted to cheat, but this will result in a scrunchie that looks small and flat rather than full and ruffled. When you are done, it is time to join the two ends into a doughnut shape. You're going to use this yarn to sew up the edges.  Thread a blunt knitting needle onto the end of the yarn and sew the two edges together. Make sure that the stitches on both rounds line up so that you get a smooth join. Tie a knot in the end of the yarn and break it off. Leave about 4 inches (10 cm) of yarn. You can do this with the yarn needle or the hook. Once you see how easy it is, you'll be making lots more!

Summary:
Check the tube length. Break off the yarn, leaving about 15 inches (40 cm) of yarn from the hook. Pull the remaining thread into the inside of the scrunchie. Enjoy wearing your scrunchie, or wrap it up and give as a gift.