After you download a map you will see a .zip file. Inside this file (archive) is a .vpk file which needs to be installed.
If you need help extracting the .vpk file, or if you can not open the .zip/.rar file, download and install winrar.

Double click the .vpk file to have Steam automatically install the custom map / campaign. If you are prompted to select what program to open the file with; Right click on the .vpk and select "Open With...". On the next window press "Browse" and look for Steam's 'addoninstaller' in your left 4 dead installation directory.
\Steam\steamapps\common\left 4 dead 2\bin\addoninstaller

To manually install add-ons (.vpk), follow the steps below.
Navigate to your left 4 dead or left 4 dead 2 directory. If this folder does not exist, create this folder.
\Steam\steamapps\common\left 4 dead 2\left4dead2\addons\

Once you have found / created your addons folder; copy and paste the .vpk files into this folder.
After your .vpk file has been copied to this folder it has been successfully installed.

Once you have successfully installed your maps you can enable or disable them at any time.

1. Open L4D / L4D2.
2. Click on 'Extras' button from the game menu.
3. Click on the 'Add-ons' option to load all installed addons.
4. From here your list of addons will be displayed. Check / uncheck addons to enable / disable.

You can remove custom maps by deleting them from your /addons/ folder.