Figure out what size of fabric you need to cut. You can make it as long as you want. The width should be 2x the desired width, plus 1".
Cut your fabric based on the size you want.
Fold the fabric rectangle so that the long sides are touching, and the good sides are facing each other.
Sew along the long edge, stretching as you sew. As you finish, leave a long chain - about 6" - trailing off the end.
Turn the scrunchie so that the right side is facing out. Tie the end of the trailing chain to the loop of a drawstring threader. If you don’t have a drawstring threader, tie it around the head of a crochet hook, or through a large safety pin.
Run the tied off end through the length of the tube, starting at the end to which the chain / cord is attached.
Pull the threader through the opposite end. As you go, gently ease the bunching tube off the back end of the needle, and onto itself. Be very careful not to break the thread / cording!
Run a length of elastic - I use ⅜" braided elastic - through the scrunchie, bunching the fabric as you go.
Tie the two ends of elastic together a couple times so that it forms a loop of the desired length.
Straighten out the fabric so that the seam line forms a continuous path around the elastic - I like to position it facing out.
With one edge of fabric overlapping the other, either sew through all layers with your machine or hand sew around the opening.