If you're only used to downloading things from the exchange and don't have an idea what a package file is... then read on.

TOOLS NEEDED:
Delphy's Install Helper Monkey
Winrar

Now to download custom content off of creator sites, you will need Winrar - it is manditory. It lets you unzip .rar and .zip files onto your computer. This is not just made for The Sims content but for many other downloads on the internet. You can download Winrar HERE! for free. Once you have that downloaded, you're ready for the next step.

Most of the custom content that users create are package files and the only way for them to work is with a simple program that does all the work for you. Meet the Install Helper Monkey by Delphy. This mod automatically activates the .package files so they can work in your game. The only thing you need to remember is that whenever you wish to download a .package file (something other than Sims and lots) you MUST download it in your root directory of your game in the Packages Folder under Mods. So it'd be something like this:

C:\Program Files\Electronic Arts\The Sims 3\Mods\Packages

If you download them anywhere else, they will not work. So once you downloaded the file into that folder, right click on it and click "Extract here..." that will take all the files in the .zip or .rar file you downloaded, and open them up into the Mods/Packages folder. So it will now show up as a .package file with a nice little monkey's face on it.

Start up your game and see how it goes! Make sure to read instructions on custom content you wish to download because there may be important information you cannot miss.