Download the L4D SDK Documentation.

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

It includes some example vmfs that you should rip apart and see how they are made.

My Notes:  

I. Introduction
a) Light Source
i. Different models mean different types of light should be used
ie: Flouresecent bulbs sometime give off a blue or yellow hue light.
ii. How power should the light be?
ie: A Light bulb is not going to light an entire factory
II. Making the Entites
a) Prop
i. Prop_static
ii. Change world model
b) Glow
i. env_sprite
ii. Change sprite name
iii. Change render mode to World Space Glow
c) Light
i. light
ii. Change Brightness
iii. Change Falloff
iv. light_spot
v. Change Brightness
vi. Change Constant
vii. Change Angle
III. Tip
a) Group
i. Select All entities
ii. Hit Ctrl+G