Instructions
Download one of the many different "Unreal Tournament" mods available for the PS3 from the UT3 modding website. As "Unreal Tournament" is a community developed game, all of them are free.
Plug a USB thumb drive into your computer's USB port and save the mod to the USB thumb drive. This is done by dragging the downloaded folder, which will be named "PS3," directly onto the thumb drive.
Pull the thumb drive out of the computer and insert it into the PS3. Start the "Unreal Tournament" on the PS3 and select the "Community" option on the main menu. Select "My Content" on the next screen.
Press the "Square" button on the PS3 controller and select the "USB Device" from the next screen. Select "Yes" when asked "Do you want to import the data?" The mod is now installed.