How to make an xorg.conf file

I just installed Ubuntu 12.10 and I need to make a custom xorg.conf file. By default, there is not an xorg.conf file on my system. I know what I need to write in the config, but I just don't know how to make X load it...

Should I put my custom commands in a file that already exists? Where is it? Should I make my own xorg.conf file and put it in /etc/X11?

If you would want full and proper xorg.conf file (tested on Ubuntu 12.10):

1. Switch to console mode: Ctrl+Alt+F1

2. Kill x server:

   sudo service lightdm stop
   

3. Generate new xorg.conf file:

   sudo X -configure
   

   This will create xorg.conf.new file in your current directory.

4. Rename and move:

   sudo mv xorg.conf.new /etc/X11/xorg.conf
   

5. Return to GUI:

   sudo start lightdm
   

taken from Linux Mint forums (go to point 3. In Mint, xorg.conf is not present by default...)