Find a trusted Sims 4 mod website or creator page
Download the mod file to your computer
If the file is zipped, extract it
Open the Sims 4 Mods folder
Move 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 mod list on startup