To fasten the chin strap, it is best to begin from a standing or sitting position.
Lift the chin strap so that the base rests against your chin, it should mainly support the underside of your jaw and extend over the front of your chin, to a little below the mouth.
Lift the 2 straps and secure them to the upper rear half of the head, ensure the device is tight enough that it holds your jaw in place, without feeling constricting or uncomfortable.
Once secured, adjust the side straps so that your ears are through the holes, if required.
When correctly fastened, you should feel the strap is holding your jaw closed and slightly forward.