Step 1: Open Up Steam
Step 2: Enter Your Settings
If you want to make a counterstrike source map, use the ep1 engine and choose the game counterstrike source.
For me, I'm going to use the ep2 engine.
When your done choosing the settings, doubleclick "Hammer Editor" at the top.
Step 3: Click on the brush tool
To start making the base of the map, click the brush tool. This is the small square icon in middle left.
Step 4: Click and drag a box
Once you press enter, press Control+H. A window should pop up. In the input, put in "-12" (negative 12). Then press enter.