Download the mod file from a trusted Sims 4 mod site or creator page
Check that the mod is compatible with your game version
If the file is zipped, extract it
Open the Sims 4 Mods folder
Windows path: Documents > Electronic Arts > The Sims 4 > Mods
Mac path: Documents > Electronic Arts > The Sims 4 > Mods
Place the mod files into the Mods folder
Keep script mods no more than one folder deep
Open Sims 4
Go to Game Options
Open Other
Enable Custom Content and Mods
Enable Script Mods Allowed
Restart the game
Verify the mod appears in the mod list on startup
Update mods after game patches
Remove broken mods if the game has issues
Back up your Mods folder before major updates