Gopher's video guide for installing and using the Nexus Mod Manager is helpful for learning how to use it, and the mod manager is really sleek and intuitive once you're familiar with it. It also allows you to set your own mod load order (helpful for preventing mod conflicts), and enables virtual installs so that you're not directly replacing game files (less tampering with main game files means less chance of screwing something up and causing critical errors). From there, it's simply a matter of downloading and installing your desired mods - you can do this manually, typically by extracting files and folders and placing them directly in your game's main install folder, but I'd recommend using the Nexus Mod Manager because it makes keeping track of your mods much more convenient. It's pretty simple, really, and only involves editing the game's ini file.
If this is your first time modding Fallout 4, note that you'll have to tweak some settings to enable unofficial mods (basically anything that you aren't paying for directly through Bethesda's in-game store).
