Fasten the top of the chin strap into the snaps above the small holes above the ears. Pull the bottom part of the chin strap underneath the chin. You can adjust the strap for fitting purposes by lengthening or shortening it from the adjusting clip.
Place the bottom of the chin strap into the snaps below the holes that lie underneath the ears. The cup on the chin strap should be centered and fit firmly under chin. Adjust, as necessary.
Check to see if the helmet fits snugly on the head. If not, adjust the chin strap until the desired fit has been reached. The helmet shouldn't hurt your head, it should feel like a regular snug baseball cap on the head.