Hi rileybk, there are quite a number of phones called "Samsung Galaxy S2". Have a look at the Wikipedia here: https://en.wikipedia.org/wiki/Samsung_Ga...
The device shown here seems to be a SGH-I777 which you can read on the label below the battery.
As Wikipedia describes these devices can be very different from each other so that your replacement part might not fit or work at all.
(btw, I came by to look for a guide for the so-called "international version" i9100 ... I have to see whether this guide will help me nethertheless)