Cut the length of elastic needed for your head. For me, this was 22 inches. I lightly stretched the plastic around my head, and then added about 1cm for seam allowance. If you’re worried about size, I’d go tighter than you think you need. Mine ended up a bit loose, but they’re still usable.
Sew the ends of the elastic together to form a single loop. The stitch doesn’t particularly matter here.
Use the pen or permanent marker to mark the equidistant points along the edge of the shower cap. Start by laying the circle flat and folding it in half to make a semi-circle. Mark the two side points. Bring those points together to create another semi-circle. Mark the new side points. You should now have a circle with 4 points marked (basically a unit circle from geometry class). Between each set of marks, add another point so you have 8 total around the cap. I went on to do another round of marks, splitting the circle in 1/16ths, but this is optional.