First of all, you must open Hammer, so you can begin to map. The first time you open Hammer 4.0, it will automatically configure all the settings. This is a huge advantage to beginners, as setting up the previous Hammer was at least half the battle when it comes down to mapping.
Go to File > New then hold the "[" button until the grid size gets no smaller. This makes mapping a lot more accurate, (but your personal preference may vary here) and zoom in using the mouse wheel. You will see there are 4 different screens. In the top left view, click on the word camera and choose "3D Textured".
Select the tool for creating a brush and drag a box across a small section of the map and press the enter button, then adjust its size so that it's dimensions are 144x104x16 units as shown in the image below.
Don’t worry if you are using a different texture or if the texture doesn’t align. We are just trying to get the very basics done just now. Make another 3 brushes and arrange them so they look like this:
Make sure that all 4 brushes meet without any gaps or else there will be problems with compiling.
Next we need to make a floor by simply adding a 5th brush that covers the area between the 4 walls. Once again select the brush tool (brush.jpeg) and drag across the area shown in the pic below.
Your map should now look like the above image. We now need a player spawn, so select the entity tool (entity.jpeg) and click inside the walls. A box should appear. By default this entity will be called info_player_terrorist, which is the entity that we want right now. Your map should now look like the image below.
We now need to make a roof for this box room, so select the brush tool again (brush.jpeg) and drag across the upper section of the walls so it looks like the image below:
To complete the map we now need a light source, so select the entity tool (entity.jpeg) a click inside the box, but in the upper section. While the entity is highlighted, press alt+enter, which will bring up its properties. Where is says, "Class:" select "light" from the drop down menu. Select "Brightness" from the "keyvalues" menu and fill it in as shown below and press enter.
The 1st 3 digit number stands for Red, the 2nd for green, 3rd for blue and the 4th stands for the brightness. You will notice that the entity that you made a light is no longer displayed in the camera view as a box, but now look like a light bulb. The map is now ready for compiling, but before you begin to compile, make sure your Hammer Editor looks like the image below.
If it matches the above image, press F9. Doing this will cause the save box to appear. Call your map box_test and press save. You will then see the compile dialog box appear. Fill it out so it matches the image below then click on ok.
After pressing Ok, the compile window will appear. This shows the procedures that the tools are performing to compile your map. You will know that the process is finished because it will have "Copy to Clipboard" at the bottom. Your map is now complete. By default, Hammer will have created the .bsp file in this folder - C:\Program Files\Valve\Steam\SteamApps\email@example.com\sourcesdk\cstrike_sample_content\maps.
You need the map to be in this folder to be able to play it - C:\Program Files\Valve\Steam\SteamApps\firstname.lastname@example.org\counter-strike source\cstrike\maps.
You can edit where the .bsp file is generated to by going into Tools > Options, but if you are a beginner, it is advised that you just copy and paste the .bsp file to the appropriate folder.
To test the map, open Counter-Strike: Source, go to "Create Server" and select your map name. It should look like the image below in game.
Congratulations. Please note that this is a very basic map for beginners.