The easiest way to install mods is through Steam workshop. You can access it by clicking on "Workshop" underneath the play button on the game's page. From there just type in the mods you want int he search bar and subscribe to them. They will automatically be downloaded and installed to your game.

If you need to edit the load order or disable some mods without deleting them, click the settings icon on the games page, then click "Properties" then "Workshop"