The iPhone's battery wasn't holding a charge long as it used to
The repair was easy. The only tricky part was trying to remove the adhesive strip because it was easily breakable. The guide was pretty straight forward. I purchased the battery from iFixit, and the first battery I received was all out of wack and couldn't charge pass 17 percent, but they quickly sent me a new one and the new battery was perfect. Overall, iFixit is a great company with awesome customer service and provide a super easy step by step guide in fixing my iPhone.
For the adhesive strip, my advice is only: be patient and take your time. Also use your hands and not tweezers or pliers. If you do snag, don't fret. Try to get the other adhesive strips, then you need a heating pad to proceed heating the back surface to easily pry out the battery. Another thing is if sliding the two adhesive strips to the side is too hard, just try to pull it out straight. It's meant to be pulled out straight anyway.
As to fixing any iPhone or MacBook in general, make sure you work on a "rubbery" surface. I did mine on an old mouse pad. This way the screws you may drop can ricochet all over the place and you may lose that screw (also have a handy magnet to recover lost screws like grab one from your fridge) Secondly, what helped me was to print out the guides and get double sided tape. What I like to do is that I place the double sided tape on top of the image where they indicate what screw to remove. After I remove that particular screw, I stick that screw on the image so I know exactly which screws go where. Thidly, try to avoid "prying" when the pivot point is directly onto the surface of the superficial side of the product. For example, if you're prying something and the pivot point is resting on the outside or edge of the aluminum unibody, stop! Figure out a different pivot point. Lastly, be patient. Take your time. If you start to get frustrated, go for a walk, avoid coffee, and resume when you're calm.