What you have to know to start creating your own theme.
This guide is written for that people (mainly designers) who wants to create their own icon theme for the GNOME Desktop Environment.
The first and best thing to do when trying to create a new theme is look to an already working theme (look to "$HOME/.icons/UltimateGnome", or to "/usr/share/icons/gnome") and see what are the files which compose a GNOME icon theme.
You'll see that there is a file called "index.theme" where you could write the most important information about the theme. Than there are a lot of folders which contain .png files .svg files and sometimes .icon files.
The first thing you could to learn how to create a theme is to try to change some icons (maybe you could start from the "folder/directory" icon), and see what effects it takes on your desktop.
You'll see that the icons of a theme show up correctly on your desktop only if their names are correct.
For more informations look at other pages here on this website, or read the GNOME icon theme specification (also here) that is the main document about icon themes on GNOME.