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:
You can always post a question on the site, or a request for more details, and a fellow user may be able to help.
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:
