Install Sims 2 Mods

This post will give you the quick and dirty basics of installing a hack, mod, or custom content.

Steps

Download a mod, or hack, from any of the various Sims 2 websites. Save it to a safe place on your computer where you will remember to find it, such as the Desktop or a custom subfolder of the My Documents
Extract the downloaded file from its compressed form (usually .zip or .rar files). The file will now be a .package file, a .bak file, or a .sims2pack file. It may also include .jpeg, .gif, or .bmp files (which can be deleted), or a .doc or .txt file, which should be opened & read thoroughly for further instructions about the mod or hack.
IF THE FILE IS A .PACKAGE FILE OR A .BAK FILE: Put it into the Sims 2 "Downloads" folder. This folder is usually going to be located under My Documents\EA Games\The Sims 2 in your computer. If you have found that location & there is no folder named "Downloads", you can go ahead and create one.
IF THE FILE IS A .SIMS2PACK FILE: Double click on it. A blue window will pop up asking you to install the content. Click OK or Accept or Yes or whichever option the installer has.
Congratulations, the content should now be installed.

First you need to have a Downloads in this directory: Documents/EA Games/The Sims 2/ If you already have one don't bother making an extra.(Make sure it is spelled correctly with a capital D).
Become a member of modthesims.com or any other custom content site, but I recommend modthesims.
Under the Downloads tab, on the site, scroll down to download. Some clothes require meshes that may have to be downloaded as well for it to show up in the game.
Most of them are zip files, so download winrar or 7zip to extract them to your Downloads folder.
When you first play your game after adding custom content, it should ask you if you want to enable disable the custom content. Choose enable. You will have to restart the game to see changes.
Now have fun with your stylish Sims!

You must have custom content enabled in your game for hacks or mods to work properly.
Always read the instructions that come with hacks and mods. Not all hacks and mods are compatible with each other, and some of them require a special method of installation to work properly.

Always run a virus scan on hack and mod packages before extracting them from their .zip or .rar form.
Back up your game before installing any new hacks or mods. They are all 3rd party content, unsupported by the creators of the game, and as such may reduce your game to a flaming ball of nothing.