Summarize the following:
Take a belt that fits well – either one that you already have or go to a clothing store and try on belts until you find one that fits the way you want it to. You want the belt to have several notches on each side of the notch you use. A belt that fits well won’t use the last notch or the first notch when it closes. Be sure to wear pants that fit you well so that you can get an accurate idea about what size belt you need if you’re trying on a variety of belts. Put the belt on a flat surface like a table or on the floor. Lay it down so that the belt is completely flat and doesn’t have any bumps sticking out. Make sure the belt is fully extended and doesn’t have any slack. If the belt doesn’t want to stay flat (because of overuse, for example), you can place a heavy object on both ends of the belt to keep it stationary. Grab a retractable measuring tape or a fabric measuring tape. Measure from the base of the buckle prong to the center hole. If you don’t use the center hole, measure from the base of the buckle prong to the hole you use the most. This number will probably range between 30 inches (76.2 cm) and 60 inches (152.4 cm), depending on your waist size. The number (in inches) that you found for the measurement of the belt will be the size you use for ordering your new belt. For example, if the measurement number is 34 inches (86.4 cm), order a size 34 belt.  If you are using the last hole on a belt, consider moving up a belt size (in this example, to a 36) so that there is room to adjust the belt in the future. A correctly fitted belt is usually sized to the center hole. If you are using the first hole on the belt, consider moving down a belt size to a 32. Alternatively, you can use a pair of jeans that fits you (one that is measured numerically by waist size in inches) and use that measurement to determine your belt size. Simply add two inches to the size of the pants and use that as your belt size. Remember that the pants must already be sized in inches, otherwise this method won’t be effective.

Summary:
Find a belt that fits. Spread the belt out flat. Measure the belt. Use the measurement that you found to order a belt. Use an existing pair of pants.