Suspenders are long strips of fabric worn over the shoulders to hold up your pants. They are attached to your pants by clips or buttons and have an "X" or "Y" shape at the back to provide extra strength and support and to prevent either shoulder strap from falling off your shoulders.
The easiest way to wear suspenders is to first attach the back straps. Use adjustable straps, preferably with the back panel (the part where the front and back straps come together) placed between the shoulder blades for maximum comfort. After attaching the back straps to the pants, pull the front straps over the shoulders and fasten them.
The rule of thumb is to wear them vertically across your chest. Adjust them so that they will keep your pants up but still fit comfortably. If done properly, they should always feel less than a belt.
The adjusters on the back and front straps will help you maximize the adjustment to achieve ultimate comfort.