Patch your game
It’s absolutely important that you have the latest patch installed, otherwise you will not be able to install downloads from SocialSims.net or they won’t behave as expected in game. Go to the Updates tab in the Game Launcher and patch your game from there.
In case you get a ZIP after download, use the built-in ZIP handling in the operating system to extract the Sims3Packs from the archive(s). If you need more information on this, check this help page by Microsoft.
Double-click the Sims3Pack you have downloaded and it will install into the game.
If you have downloaded a lot of Sims3Packs, place them into:
Documents/Electronic Arts/The Sims 3/Downloads (PC & Mac)
If you downloaded ZIPs, you first need to extract the Sims3Packs from it, then place it in the folder mentioned above.
Start the Game Launcher and go to the Downloads tab. Select the items you want to have in your game and click Install.
Remember to uninstall an item first before you reinstall it.
In the game you recognize the custom content by the shopping bag.