Start off in your map, I created a small map for myself, with a terrorist for reference, and Developer textures for dimensions. Refer to screen shot 2.
Find a suitable texture for your walls, I chose a brick texture, that has a small detail on the bottom, its good to choose textures that are not just plain colors. Refer to screen shot 3.
Once you find a suitable texture for your wall, apply it to the wall. Now that you have a decent wall texture, its time to add your first brushwork improvement to it. If your texture has details like the brick I chose, at the bottom of the texture, then create a brush over that piece of texture. Refer to screen shot 4.
Turning a 2d surface into a 3d surface is always good for improving brushwork. It makes it stand out more. Make sure to apply the nodraw texture to any part of the brush that players will not see while playing in your map. This will improve FPS in the map.
Once you have successfully created a small detail on your wall texture, it is time to make further details to the wall. I chose a wood texture for this part of the tutorial. Add any brushes to the already existing wall, like I have. This will make the wall seem even more detailed by adding small pieces of wood to it, think of it as supporting the wall up. Also make small details into Func_Details by pressing Ctrl + T, this will optimize the map further. Be sure to nodraw the back face of the texture. Refer to screen shot 5.
If you wish to add further details to that wall, you may. I added a small window with a glass pane. You can use the clipping tool to make specific cuts in brushwork. Pressing Shift + X to toggle the clipping tool, and dragging the tool in the top, bottom, or side windows of the brush, and pressing enter to cut into that brush.
Once you've improved the brushwork on your wall, you can move on to different areas of your map, Like the ceiling. I added wood frame work to the ceiling to make it look like the ceiling will not cave in, it is being supported. Make sure to choose textures that would look realistic. Refer to screen shot 6.
Editors tip: Pressing Shift+X repeatedly while using the clipping tool will change how the brush will be cut, red is where the brush is deleted, white is where it saves the brush for more complex cutting, you can delete the pieces individually.