Don't worry, there are several places online that can help you out with a routing serpentine diagram for a V6 Mustang. I found the picture below on fixya.com, a useful forum for anyone missing a manual or a technical picture:
If this isn't what you're looking for, serpentinebeltdiagrams.com is a more specialist site, and has serpentine routing pictures for the 2004 Mustang 3.8L.
allfordmustangs.com is another forum, operated and supported by Mustang enthusiasts where, once you've joined up, you can post a request for a serpentine diagram in a thread dedicated to 1979-2004 V6 Mustang queries. A more generalized car community can be found at greatautohelp.com.
YouTube may seem an unlikely place to find help with mechanical issues, but this film shows a user installing and replacing a serpentine drive belt for a 3.8L Mustang: