Download the mod file from a trusted Sims 4 mod site
If the file is zipped, extract it
Open the Sims 4 Mods folder
Windows: Documents > Electronic Arts > The Sims 4 > Mods
Mac: 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
Select Other
Enable Custom Content and Mods
Enable Script Mods Allowed
Restart the game
Check that the mod appears in the Mods list on startup