Instructions
Find a Rivet City house mod. You can find them at Fallout 3 Nexus or Planet Fallout (see Resources), among similar websites.
Unzip the mod using an archive program, such as 7-Zip or WinZip.
Place the mod files in the "Data" folder of your "Fallout 3" directory. Your directory will vary based on where you installed the game and whether it is the Steam version of the game. It is usually found at: C:\Program Files\Bethesda Softworks\Fallout 3\Data.
Download and install "Fallout Mod Manager" (FOMM). It can be found at the same websites referenced in Step 1.
Launch "Fallout 3" by double clicking the FOMM icon on your desktop.
Make sure the box next to your Rivet City housing mod is checked.
Click "Launch" to launch the game.
Follow the mod's instructions for acquiring your new house in Rivet City.