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This tutorial outlines the some of the main components that make-up a left 4 dead map

Cubemaps

You will notice that there are these reflective spheres placed throughout the tutorial_standards level:

Note: To get the information for the cubemaps, they need to be built in the game. Load up your level in the game and type "buildcubemaps" and press the Enter key to build the cubemaps. This will change the bsp so make sure it isn't read-only.

For more information about cubemaps, click on this link:

http://developer.valvesoftware.com/wiki/Cubemaps

Overlays

Overlays are sheets of materials that you can place on top of a brush. We used overlays to show dirt/trash/rugs on the ground, graffiti/signs on the walls, and door frames.

Let's start with an easy placement for one on the ground.

The room with the staircase has an example of trash on the ground.

To place an overlay, click on the 9^th icon down on the Tool Bar, the Apply Overlays Tool.

Click on the Browse Button on the Texture box to select an overlay texture.

Type "decal" in the Filter Field and find the texture "decals/trashdecal01a"

Point to the ground in the Camera viewport and left-click to place the overlay.

 

Press Esc to go to the Selection Tool and you can move/rotate/scale the overlay similar to brushes.

Overlays have assigned brush faces. If you move an overlay to another brush, you will need to assign the brush that it is on top of to it.

Move the overlay to the hallway behind the door shown in the image above. You will notice that the texture does not appear anymore on the floor.

Now open its properties by pressing Alt+Enter.

Click on the "Brush faces" row and click on the "Pick…" Button on the right side.

 

With the properties still open, left-click on the floor that the overlay is now on top of in the Camera viewport.

You should see a brush face number appear in the row that is different from the original.

Click on Apply and close the properties to see the overlay on the hallway floor.

Now, take a look at the door frame in the first room of the tutorial_standards map.

You will see an overlay that has multiple brush faces assigned to it. When you click on the Pick… Button, you can assign multiple brush faces to have the overlay appear on top of each brush.

Creating an Infected-Only Ladder

Creating an Infected-Only ladder is much the same as creating a ladder for everyone to use.

 

Texture the brush with the tools/toolsnodraw texture.

Enter the Texture application mode and select the front face of the ladder.

Click on the Browse button in the Face Edit Sheet.

Click Apply to apply it to the front face.

Select the brush and use Ctrl+T to make it a brush entity.

Change it to a func_ladder in the Class drop-down list.

Save your map, compile it, and run it in the game.

In nav_edit mode, point at the ladder with your cursor and use "nav_build_ladder" from the console or a bound key (the L key has been bound to nav_build_ladder in the nav_cfg file).

Note: Only the common infected and the Tank can jump and climb up to a height of 180 units or less. Hunters can leap when they are spawned a great distance. However, Smokers, Boomers, and Hunters (that are in ghost mode) must use infected ladders or survivor ladders to get over some obstacles.

That's it for this tutorial 

hope it help

Welshy/Valve

Source http://www.l4d.com/LevelDesignDocs.zip