If it is a simple One Version Mod: Copy the *.package File into you Sims 4 Mods Folder (Documents > Electronic Arts > The Sims 4 > Mods). If a Mod has Optional Addons, install them the same Way.

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If it is a Mod which has more than one Version: Copy the *.package File you want into your Sims 4 Mods Folder. Dont use more than One Version at the same Time!

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If it is a Script Mod (has a *.ts4script inside the ZIP File): Copy the *.package File & *.ts4script File want into you Sims 4 Mods Folder (not more than one Subfolder deep for example Documents > Electronic Arts > The Sims 4 > Mods > ScriptMods). Dont rename any of those or it might not work. Enable Script Mods/CC via Game Options > Other Ingame.