Design your chinstrap to make sure you get the length and dimensions correct. If you are making a chinstrap for a football helmet, measure the number of inches from one snap to the other. (Make sure you allow the tape measure to curve naturally to simulate the room the strap will need to accommodate your chin.) Mark the length you need for your chinstrap on a piece of scrap paper and draw the shape of the strap so you can be sure the chinpiece will be big enough to hold securely.
Mark this pattern on a piece of durable material. Leather is relatively durable and easy to work with. According to Corps of Discovery, you should always, "make your garments with the rough side out." Vinyl and even sheet plastic can be used, as well. Make your decision based upon the hat the chinstrap will be attached to. A football helmet requires a stronger leather or canvas, while a cowboy hat would look best with thin leather straps.
Cut the pattern out of the material. Leave a little bit extra around where you marked to allow for a margin of error. Remember; you can always cut a little extra, but you can't put material back on once you've cut it.
Finish the edges of the chinstrap for better aesthetics and comfort. This can be as easy as looping around the fringe of the chinstrap to keep the edge smooth. You can also sew a thin layer of canvas or another fabric around the edge.
Decide how you will attach the chinstrap to the hat. If it's a football helmet, you can use a mallet to put a metal snap into the end. (If this is the case, put the helmet on your head and mark where you'd like to put the straps so you get a good fit.) Attach the chinstrap like a belt by using an awl to make a series of holes in the chinstrap that will allow adjustable connection to a buckle.