Download a map on the main page of the website. Or from the below links.

City 17 Dead Before Dawn
Suicide Blitz Precinct 84
I hate Mountains Night Terror
Dead Vacation Redemption
Dead City Pit Stop
The Parish Cold Stream
Silent Hill Death Abroad
City of the Dead Dark Blood
Naniwaw City One 4 Nine

Download the extraction file program here and install it.

 

Open the map you have downloaded. This will most likely be in your Download Directory. (This is a free copy) of the extraction program. When it asks you to purchase it, just click the close button.

 

Click the "Extract To" Option

 

Direct your extraction to your Left4Dead addons folder. (See the image below) Your addons folder may be located somewhere else. But this is the most common location. The addons folder holds "VPK files" - this is the format of the custom maps.

 

You have now installed a custom map. "Restart Left4Dead" - Done!!