Simply drop the RFMOD file you have downloaded into the Packages directory and then use either Option 1 (outside of the software) or Option 2 (inside the software) to install it.
By default, the packages directory is located at /My Documents/rFactor2/Packages/ and you may wish to include the /My Documents/rFactor2/ directory in your regular backup schedule.
Option 1: You may want to configure which content RFMOD packages (cars and tracks) are active before you launch rFactor 2 for the first time. To do this, click the Manage Mods button at the bottom-right of the Simulate tab on the launcher. A new window will open and it will automatically read your packages directory for RFMOD files. Right-click on any mod in this window to see options available to you.
Option 2: Launch rFactor 2 into single or multiplayer and you will see either a series option (single player) or mod management option (multiplayer). The software will automatically read your packages directory for available mods, and you can click install or update right inside the software