Quadrilateral Cowboy

Select and download one of the mods
Navigate to the QC installation path, where qc.exe is located (there should be a base folder there too)
Extract the folder from the zip to this location, keeping all the content inside the folder (e.g. the cheese folder will be next to base and qc.exe)
If Quadrilateral Cowboy is already open, restart the game
On the main menu, go to Mods and double click mod from the list
After the game reloads, click Career > Start