1. search through images on the web for a good background image.
2. upload your own background image if you have a picture or your own design you'd like to use.
I've used a lot of layout generators and tested them all out, and my recommendation is www.backgroundstation.com. It has proved to be the simplest and the best quality of all of the myspace layout generators that I have tried.
There are four easy steps to creating your layout:

You can choose from several well designed themes, we'll talk more about that later.
The background image is the real meat of the layout - use your own or search yahoo, google, and flickr.
There are some options to tweak your layout and make it even more unique.
Send it to myspace in a flash! You're done!