I cut the shirt into strips of 20 inches long and 4-5 inches wide. I didn't worry about perfection so my lines were a little off. I folded them in half longways, and sewed the edges together lengthwise. Then I turned the material so it was right side in, some people use a pencil to help with this.
I took between 5-6 inches of elastic and weaved it through the inside, you can do whatever length you want depending on the thickness of your hair and how much expansion you will need. I tied the elastic in a knot after weaving it through. Then you finish sew the two ends together, and they should look something like this.
Pretty easy and very forgiving of mistakes. A perfect project to do with first time sewers. I will surely be doing this during the summer with my gang of older babysitting kids.