Before starting make sure that Your game is updated to the latest patch!
And that You've deleted previous files of the MOD you're going to install.
After downloading the mod, Right click on the ZIP file & select "Extract Here"
Select the mods "folder" OR "files" then right click on them & select "Cut"
Go to Your "Sims 4" Mods folder found in " Documents > Electronic Arts > The Sims 4 > Mods"
Right click then select "Paste"
This is how the folder should look like in Your "Mods" folder.
-DON'T BURY THE FOLDER INSIDE ANOTHER SUB FOLDER
Make sure that "Mods" and "Script Mods" are enabled in Your game's options!
A game restart might be required.