This explanation is for The Sims 4 PC verison only!
Location on your PC:
Documents > Electronic Arts > The Sims 4
In the Sims 4 folder/directory make a new folder and name it Mods.
Now open your Notepad and copy the below text and paste on notepad.
Now save that as Resource.cfg and place it with in your Mod folder you just made.
Now You can add mods, All package files you download need to be placed within your mod folder and you can make new folders to keep it tidy per creator or how ever you wish to keep it organized. You can have up to 6 directories with in each folder with in the mod folder.