How to make a custom map in Left 4 Dead

This is a tutorial for those new to custom map design in Left 4 Dead. It should also be useful to some experienced source mappers who aren't fully experienced with mapping for Left 4 Dead.

It basically makes some recommendations for making a map more fun or interesting through dynamic use of triggers, physics, and color correction among other things.

Important tip:
Remember that each map needs an info_director and to use nav_generate in the console after compiling the map to generate a navigation network for zombies.