Press to select the Installer icon on your SpringBoard.
Press to select the Install tab at the bottom of the screen.
Press to select Themes (Customize) from the list of Categories.
Press to select a Theme from the list of Packages. I selected Designer Image Set form the purposes of this tutorial.
Press the Install button at the top right of your screen.
Press the large Install button that appears.
When the installation completes you will be returned to the Categories list. Press the Home button to return to your SpringBoard.
Press to select the Customize button.
We are now presented with all the different items we can customize. To see the new images we installed lets press the Carrier Image button.
Notice now that we have a bunch of new Carrier Images to select from.
Lets try and change an image to demonstrate how this works. From the Customize menu select WiFi Images.
Notice we now a few different WiFi images to select from. For fun lets select the Traffic Light Image Set.
Press the large Ok button that appears to confirm installation.
Finally, press the Home to see the new image on our SpringBoard.
You can install many different Themes from the Installer app and customize your iPhone to your hearts content. Each button on the Customize menu will let you change different interface items on your iPhone.
I will write a tutorial on how to create your own Customize theme in the near future.