This is useful stuff for making ties for masks in place of elastic. I’m not sure it’s comfortable for behind the ears but it’s great for ties.
The sort of shirt without side seams is preferable. Cut off the bottom from just under the armpits.
Cut off the hem and trim the shirt to be an even rectangle. I used chalk to mark lines 1" apart from side seam to side seam.
Using scissors or a rotary cutter, cut the lines started at one side and stopping about 2" from the other side as shown. It is important NOT to cut all the way through.
This step is how you get a continuous strand of yarn. Unfold the side you did not cut through and mark lines as shown. Cut along the lines.
There will be a small jog as seen here.
This is the magical part: starting at one end, firmly grasp the yarn in both hands and stretch it, pulling the yarn through your hands from one end to the other. This causes the edges to curl up into a rounded yarn shape. I got yards and yards of it from just one shirt!