To install custom content, you’ll need to find the Mods folder for your game. The default file path for this folder is:
My Documents > Electronic Arts > The Sims 4 > Mods
This is where you’ll be dropping your custom content. Note that many content creators compress their creations as ZIP or RAR files to make sharing easier, so get one of these free file extractors to unpack your content before you place it in the folder.
Once you’ve added any mods to The Sims 4, the game will show a mod browser by default when you launch it. This will let you know exactly what the game has recognized and implemented at launch, so you can know that your files are in the right place.