My iphone quite working about 18 months ago. I was ready to dump it when I decided to see if I could fix it. I went on line to ifixit and go the step by step procedure for replacing the battery. Then I ordered the battery and tools.
The repair went very smoothly. Everything fit in place. When I was finished the iPhone came back to life. I am very happy with the results and happy with the guide provided by ifixit.
If I were doing this again, I would use a lamp with a magnifying lens. Those screws are very little. When putting the iPhone back together make sure you lift the connectors out of the way first. Lining up the SIM card slot was the most difficult.