Download the CC file from a trusted source
If the file is zipped, extract it
Open the extracted folder
Find the .package file
Copy the .package file
Go to Documents > Electronic Arts > The Sims 4 > Mods
Paste the .package file into the Mods folder
If the CC includes script files, keep the .ts4script file in the Mods folder
Do not place CC deeper than one folder level inside Mods
Open The Sims 4
Go to Game Options > Other
Enable Custom Content and Mods
Enable Script Mods Allowed if needed
Restart the game
Check the Mods list on startup to confirm the CC is installed