Summarize:

These headbands look like a wide band that is crocheted out of thin, colorful elastic. They are usually a few inches/centimeters wide. The holes will allow you to make additional rows of knotted tulle for a thicker tutu.  This method will work the best for a child's tutu. The elastic headband may not fit an older child's or adult's waist. If you really want to use this method for an adult's tutu, buy some crocheted ribbon from a fabric store, then cut and sew it into a waistband that fits you. If you only have a skinny elastic headband that is not crocheted, this method will not work for you. Refer to the elastic method instead. Because you folded the strips in half earlier, there should not be any unevenness along the bottom edge of your tutu. If you see any parts that are longer, trim then down with a pair of scissors. Don't worry about any pieces that are shorter, however; they will be hidden inside the tutu! You can also leave the tutu untrimmed for a raggedy look.
Get a crochet headband that looks nice with your tulle. Trim the tutu down, if needed.