Bias tape or bias binding is a narrow strip of fabric. It is cut at a certain angle from a larger piece of fabric, so the strip is stretchier as well as more fluid and more drapeable.
Many strips can be pieced together into a long "tape." The tape's width varies from about 1/2" to about 3" depending on applications. Many types of bias tape you can purchase have the sides folded in so the raw edges are together.
You can use bias tape to make the ties for your masks.  We recommend using about 1 inch wide bias tape, folding it in half so the raw edges are enclosed, and then sewing it shut.
A common hack is to not INSERT the ties while sewing a mask. Instead, some craftivists just stitch bias tape or fabric ties up the outside of the sides of the mask. Here is a picture from Millon Mask Challenge craftivist Runa Qaderi, who was dropping off these beauties in Ashburn, to help you to visualize this tactic.
The simple face mask pattern by the team at Johns Hopkins Medicine also recommends this technique, if you want to read some instructions.