1. Ensure you have Kerbal Space Program.
Before you add mods you need to have the original Kerbal Space Program, which is available on Steam. As of this writing it is $39.99, but Steam frequently offers it at a lower price during sales. To ensure you have Kerbal Space Program, log into Steam and navigate to the Library tab.
2. Choose a mod and download it.
There are thousands of mods, all of which enhance the game experience in a different way. Do some research and figure out what you’re looking for. Once you’ve discovered the mod you want, download it to your computer.
If you don’t know where to start looking for mods, try https://www.curseforge.com/kerbal/ksp-mods. There are 2,113 mods available to browse through. Just make sure the mod is compatible with the same version of Kerbal you have. The current version as of 9/6/2019 is v1.7.3.
3. Add the mod to the game.
In Steam, go to your library and right click on Kerbal>>Properties>>Local Files>>Browse Local Files. This will take you to the main Kerbal folder on your computer. Locate the GameData folder. Unpack your mod folder (which should be in your computer’s Downloads) into the GameData folder. Open the game to ensure your mods were installed correctly, and start playing!