A few years ago (actually 1991), I covered the plastic headband on ear muffs as they hurt my kids heads. (they are 22 and 28 now!)
I used worsted weight yarn (the kids picked the color) and a size G (4.00 mm) hook.
I just measured the length of the band when extended fully from one muff to the other, crocheted a chain that amount I single crochet, and then made it wide enough to fit over when folded. I then sc it together, leaving a tail of yarn long enough to fasten to the earmuff through the grommets. You can also whipstitch the 2 edges together with a large blunt needle.
They never complained again! :up: