Instructions
Turn on your computer and navigate to "\My Documents\Electronic Arts\The Sims 3\" on whichever hard drive houses the game. This is typically the C drive.
Create a new folder within the folder by right-clicking inside the folder and selecting "New Folder." Name this new folder "Mods" or "Custom Content."
Download the "Resource.cfg" file from any of the websites that offer custom content. "Slinky and the Sims" is one such website. You need this file in order for the game to recognize the custom content and be able to apply it to the game. Save this file in the "Mods" folder you created.
Add a new folder inside the "Mods" folder called "Packages." This folder will house all custom content files that end in the ".packages" extension. These files need their own folder separate from all other custom content you install.
Download the custom content you want. Save the files to either the "Mods" folder or the "Packages" folder. When you load your game the next time, a message will come up, letting you know that new content was added to your game.