The battery from Apple was holding about 17% of it's normal charge - the phone is pretty old at this point, so that's not surprising considering it's also a hand-me-down from my mother to me. I ordered the iFixIt Iphone 6 Battery Repair kit and it came in like 4 days, right when it said it would.
Everything was going fine until I got the adhesive strip advice part, where it tells you to pull the strips out and around the side of the battery, once the display/screen of the Iphone has been safely disconnected. I broke my strips doing so immediately, and then attempted to pry the old battery out with the tools sent instead of trying to salvage the strips with the tweezers. I even used a space heater to try and loosen the strips underneath the battery; all to no avail. Eventually I just got the tweezers and managed to get ahold of the strips again. All one needs to do is simply pull them out straight towards you at about a 60% angle, and they come out very simply. After that everything else was pretty simple; just did all the steps in reverse order to reattach everything in the correct place and put it all back together again. Fixed my pretty much unusable Iphone in just under an hour as a novice.
When pulling the adhesive strips out, DON'T PULL AROUND THE EDGE/SIDES OF THE BATTERY; I wasted around 30 minutes trying to correct that slip up. Much much easier to just pull them out straight towards you. You don't even need to disconnect the display at all, so long as you keep it secured and in place against something upright (I disconnected it anyway though, seeing as this was my first repair experience).
Also, I wish the screwdriver iFixIt sent wasn't quite so magnetic with the screws you have to unscrew and rescrew during the repair. It was hard as the dickens to put those little buggers back in to place once you got them out, because they kept jumping onto and sticking onto the screwdriver instead of slotting down into their correct holes via gravity.
I also lastly wish the iFixIt would include the steps to do everything in reverse order as the actual dismantling part of the procedure, so that customers can be sure they don't break anything upon reassembly of the device.