To install most mods, you drag the downloaded files into the “GameData” folder within the main Kerbal Space Program folder. These are generally from Curseforge, as the game links directly to there on its main menu. That being said, all the mods will still have external download links, and a few will require them as they’re not covered by CKAN’s repository. It’ll even grab any dependencies you need and suggest other add-ons to ensure you have the complete experience.
It’s a complete mod installer that searches, downloads, and updates almost all of the mods on this page. The Comprehensive Kerbal Archive Network, CKAN, is the handiest tool in your space garage.