Take out the fan shroud and mechanical cooling fan. Then release the belt tensioner with a 15mm wrench (don't quote me on the size). Pull out the old one and then route in the new one according to the diagram in the owner's manual.
I changed the belt in my car without removing the fan, so it is possible, but the fan gets in the way and it would probably be easier to take it out if you can, it was a pain working around the fan.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record