Here is the YouTube version of this tutorial, and here is the accompanying tutorial on using Canvases.
Working with the Iray render engine, and seeing what people do with it on deviantart, over time I've seen the same mistakes repeated many times. One of the most common that I see has to do with rendering night scenes or other scenes that are meant to be dark.
The thing is that the human eye is bad at seeing in the dark. If you render literally what someone would be seeing, your scene will look terrible, but this is exactly what I see many people doing. Hollywood movies often use two specific techniques to create night scenes: lit foreground objects against dark backdrops and "blue-for-night" lighting. There's a third option (used more in horror) that I will also cover, the use of strong specular highlights and/or rim lighting. Let's take them one at a time.