1. Once you have downloaded the map, you want to extract or unzip the map. 

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

3. Now that your in your SteamApps file, go into your account name.

4. Now go into your Counter Strike:Source file.

5. Now go into your cstrike file. 

6. Now have your cstrike file and the file with your map both on the screen. 

7. Now your going to drag the files for your map onto the cstrike folder. 

8. It will ask you if you want to replace or copy the folders. Hit yes to all.

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