Mods for The Sims 4 are typically downloaded as ZIP or RAR files, containing .package files. You must extract them before installing them into your game.
Extracting Files
There are a number of programs that can extract the contents of archive files. WinRAR and 7-Zip are the most popular and can be downloaded for free.
Package Files
Furniture, clothing, hair, walls, floors and almost all other custom content is stored in Package files. These need to be placed in the Mods folder, usually found here:
C:\Users\{your username}\Documents\Electronic Arts\The Sims 4\Mods
Sims & Building
Sims and Buildings are downloaded in archive files containing multiple files. All of the files in that archive must be placed in the Sims 4 Tray folder; usually found here:
C:\Users\{your username}\Documents\Electronic Arts\The Sims 4\Tray
You must extract the files before placing them in the Tray folder. Do not place the .zip file! Once placed in the Tray folder, they will appear in your in-game personal Gallery.
Creating Subfolders in the Mods folder
The Mods folder in the The Sims 4 does not allow the use of subdirectories. The instructions below will make it easier to organise your Mods folder.
If you wish to use subfolders to help sort it out, you will have to edit the resource.cfg file. To do this go to the mod folder and open the resource.cfg file in a text editor. (Save a copy of it first.) Edit it to read:
Priority 500
PackedFile *.package
PackedFile */*.package
PackedFile */*/*.package
PackedFile */*/*/*.package
PackedFile */*/*/*/*.package
PackedFile */*/*/*/*/*.packageSave the file. This will allow you a directory structure of up to 5 levels deep within the mods folder for sorting. This should be enough. (I am currently using 4 deep.)”