Well first you have to download the firmware resource editor
here
Then you must find a resource file to start with. Find one that is built for the firmware version that you want to make your theme for (such as 2.003.5 or 2.004.3, etc.). Then you can replace any of the images with custom images or images from other themes. Sometimes it is fun to take your favorite parts from a number of themes to make a new one.
It takes a little bit of trial and error to get the hang of designing your own images, but after you get used to it, it is very fun to design your own themes.
Hope this helps. And congratulations on getting an M3. I hope you come to love it as much as i love mine.
