1. Remove a screw on the iPhone, (look at photo for reference).
2. Gently pry this up from the side opposite to the battery and make sure to keep the small black piece that comes out with it, organize these in your notebook below.
3. The battery takes a lot of force! be careful this pull the tab that says authorized service provider only. This should help you out but it still takes a bit of time to loosen it from its adhesive. Do not use a screw driver to take this out, it may short the battery, a simple guitar pick will do the trick.
(Find iPhone parts here: South Mountain Repairs)