I've looked at the tutorials on installing maps and I think they are confusing. So here's a tutorial on installing map with pictures to help you understand easily.

Update Thanks to Blazeace1, he has provided a video on installing maps for servers. The video is at the bottom.

1. Once you have downloaded the map, you want to extract or unzip the map.
Shown here: http://img.photobucket.com/albums/v730/Deski/Tuts/step1.jpg

2. After you extracted or unziped, it should look something like this:http://img.photobucket.com/albums/v730/Deski/Tuts/step2.jpg

3. The next step would be going to your SteamApps file which is located at: C:\Program Files\Steam

4. Now that your in your SteamApps file, go into your account name.
Shown here:http://img.photobucket.com/albums/v730/Deski/Tuts/step3.jpg

5. Now go into your Counter Strike:Source file.
Shown here:http://img.photobucket.com/albums/v730/Deski/Tuts/step4.jpg

6. Now go into your cstrike file.
Shown here:http://img.photobucket.com/albums/v730/Deski/Tuts/step5.jpg

7. Now have your cstrike file and the file with your map both on the screen.
Shown here:http://img.photobucket.com/albums/v730/Deski/Tuts/step6.jpg

8. Now your going to drag the files for your map onto the cstrike folder.
Shown here:http://img.photobucket.com/albums/v730/Deski/Tuts/step7.jpg

9. It will ask you if you want to replace or copy the folders. Hit yes to all.
Shown here:http://img.photobucket.com/albums/v730/Deski/Tuts/step8.jpg

10. When it finishes, your map has been installed.

Hopefully this has helped you install the maps you want to enjoy. If you have any questions at all, please feel free to PM and I will get back to you as soon as possible.

Installing on a server