I installed BUFFY as a mod, then created a shortcut to load BtS with with the BUFFY mod. It may be in the forums in another place, but here's how to do it...
1. Right click on your existing BtS shortcut, select Copy
2. Right click on Desktop, select Copy Shortcut
3. Right click on new shortcut, select Properties
4. In the Properties window, select the Shortcut Tab, if not selected
5. Left click in the text box for Target:, scroll all the way to the right (or hit END key)
6. Add mod=/BUFFY-3.19.001 to the end of the text in the text box (no spaces first)
7. Select the General Tab
8. Rename the shortcut to Civ4 BtS BUFFY-3.19.001 or similar
9. Click Apply or OK to close Properties Window
Same procedure works for any mod you like to use for BtS, or Warlords HOF-2.13.003, or Civ4 HOF-1.74.003, or...