WHAT ARE THE SPECIFIC STEPS TO TAKE IN ACTIVATING A USED IPHONE ON AN EXISTING AT&T ACCOUNT?

My partner gave me her old iphone 3g since she consummated her stipulate with at&t. I already have a phone and an comment with at&t but I wish to put the iphone on my comment for my friend. Can any one discuss it me the specific stairs to activating this phone today.