Hold the tails up in one hand so that the pretied bandana hangs down, you should be able to see the sewn in elastic towards you. Now that its facing the correct direction, with your thumb and first finger hold onto the elastic by the outer edge. With a sweeping motion slide the fabric up and against your lower forehead and continue sliding fabric on till the fabric reaches your neck. Tuck in any stray hair or just leave your hair hanging down.
A good fit would be that the elastic should lay around 1” under your earlobe. Some people like to tie the tails others let them hang open. Whatever is most comfortable to you.