Instructions
Load Steam and select the Library tab. Select Tools and find "Source SDK" from the list of available options.
Launch Source SDK and choose "Counter-Strike: Source" from the drop-down menu for Current Game.
Click to load the Hammer Editor. From the Hammer editor click "File" and choose to open a new file.
Create the layout of your map using the texture tools and prop tools. Use textures and lighting to enhance the detail of your map.
Add spawn locations for players as well as other map details such as bomb spots and item locations.
Click "File" and select to save the map once your are finished editing. Save the file into your "Counter-Strike: Source" map directory and as a VMF (Valve Map Format).
Press the "Compile" button on the Hammer editor to compile the map as a BSP format. It will automatically be compiled and saved into your "CSS" directory. Close the editor.