The process is described in your manual. If you don't have a manual, I suggest you contact Befco. The C50 mower is in current production, so you shouldn't have any problem getting a manual.
Before you replace the belt, ensure the tensioner (the pulley that is connected to a spring) cannot be adjusted. On the C30 mower, the tensioner has three positions to accommodate stretching.. Belt deflection halfway between the drive and side pulleys should be ~3/8th's inch.
On the C30 (that's the model I own), the manual states to start installing the belt on the right pulley. I don't think it really matters as long as you have the belt seated in all the pulleys.
Just make sure the belt tensioner is in the first (tightest) hole when you're done.