Go to ifixit.com, look up your phone's battery replacement steps, stop half way through :)

Yeah the first two times Apple did it for me. Then Apple stopped supporting battery replacements on a phone that old, so I ordered a battery replacement kit on Amazon and did it myself, with ifixit.com's assistance.

Never again. I was genuinely shocked the thing turned on once I closed it up. It's one thing to have a conceptual understanding of how tiny the components inside a phone are. It's another thing to actually be trying to seat a plug into a socket with tweezers and just have no idea how you're supposed to tell if it's fully inserted or not.