Verified answer

Depends on what the content itself is. Custom content for TS2 will come in one of two forms: .Sims2Pack and .package files (the latter usually being contained in .zip or .rar files).

.Sims2Pack files are self-installing files, and all you need to do is double-click on them, and a window will pop up. Just follow the instructions (clicking "OK" I think twice). That's all there is to it. .Sims2Pack files are usually things like custom lots, walls and floors, although occasionally other types will be in this form.

The other file type, .package files, are typically what everything else is, and usually are contained in some sort of archive file. As such, the first thing you need to do is, if it's contained in one, to extract it. Most modern operating systems can open .zip files natively, and .rar files need a program like WinRAR (http://www.win-rar.com/ ). After extraction, simply place the .package file in the My Documents -> EA Games -> The Sims 2 -> Downloads folder (if the folder doesn't exist, create it).

Now, when you start the game up for the first time after installing the content, the game will notify you that it's detected the CC and warn you it could be harmful, and ask if you wish to enable it. Ignore the warning and chose to enable it. The only thing is, you'll need to close the game at the next chance you get (neighborhood load screen) and restart the game before the CC will appear for you. You'll also need to repeat this after installing any expansion or stuff pack.