Need a diagram on how to put a drive belt back on 42 in 16 hp craftsman

There are lots of craftsman mowers. A diagram can usually be found here: Parts & Accessories | Shop & Find Lawn & Garden, Appliance Parts at Sears PartsDirect | SearsPartsDirect.com (http://www.searspartsdirect.com)

Make sure you use OEM belts.

There are only a few ways to hold a belt in tension.

1. A dedicated tensioner
2. A friction wheel clutch
3. One of the ends the belt attaches too moves because of slots.