Step 1: Install Summerboard
First you need to install summerboard, an application that allows you install custom themes for springboard. To get to this go to installer, then to the install tab , then to the system folder to find it.
Step 2: Restart Your iPhone or iPod Touch
Step 3: Install A Theme
Step 4: Change The Theme
Step 5: Respring
Your new theme is installed with custom backrounds and icons!