Hold the guitar in a comfortable position while standing and measure the distance from the strap lug at the base of the guitar, around your shoulder, to the front strap lug. If your guitar does not have a front strap lug, measure to the headstock side of the nut.
Cut the belt to this length.
Cut a 1-inch line in the center of the belt, lengthwise, 1 inch from one end of the belt. This will be the end that fits onto the strap lug at the base of the guitar.
Cut a line in the center of the belt 1 inch from the other end. This will be the end that fits over the front strap lug. If your guitar does not have a front strap lug, cut a hole instead of a line, feed through a piece of string, and fasten the string on the headstock side of the nut, under the strings.