First, you need to select the Custom Mode at the camera Mode Dial. You might found your Dial looks different, but I'm sure you can find the C1 (or C2) icon ( In red circle) at the Dial :)
So, what does Custom Mode do? Let's begin. These are the setting for my camera, but it will be about the same as yours as well. Do not worry ^^
A Camera User Setting (or I called it Custom Mode) is actually a function to store your desire setting in Creative Zone. Let's see the setting below.
Aperture = f/5.6
Shutter Speed = Auto
Exposure = 0
ISO Speed = 3200
Picture Style = Portrait
White Balance = Tungsten Light
AF Mode = One Shot
AF Point Selection = Selectable
Drive Mode = Single
Metering Mode = Evaluative Metering
Built-in Flash = Off
These setting is my setting for a indoor shooting with Aperture-Priority Mode. Therefore, I save ALL these setting in Camera User Setting 1 (C1).
Why do I need to save these settings? It is because I might need these settings AGAIN. I need to shoot indoor photos but I'm lazy to redo all these setting. Therefore, saving all these settings in C1 (or C2), so I can simply recall these settings whenever I need them.
Camera User Setting allowed user to store any settings in Creative Zone. Therefore, when you got your "perfect" settings, you can either store in C1 or C2. The settings will be replaced if you store another settings in the same file.
However, as a photographer, remember one thing. Even you got a "perfect" setting, you are advised to test shoot before the party or dinner started. The "perfect" setting is only perfect in the previous environment, and it might not be "perfect" once you change to another environment. :)