My iPhone 6 was slowing down and I read the news of the problem with batteries and Apple's slowing down older phones with original batteries. I installed the new battery and the phone seems to be speedier and it shows 99% capacity.
I ran into difficulty with the last step of connecting the new battery terminals to the phone connection terminals due to the battery wire exiting the new battery not lining up exactly with the location of the phone terminals.It looked to be a problem of the length of the wire exiting the battery and not the alignment of the new battery within the space for the battery to sit. I worked with it for quite a while and closed up the phone but it did not turn on. I took the cover back off, worked with the terminal connections some more (even though still not in ideal alignment) and closed up the phone again. After that, it worked appropriately. I followed the instructions of discharging the battery to 10% and then charging to 100% before using the phone. I think the visual aids could better show and illustrate the wiring that exits the battery and that connects to the phone terminals, as I had already affixed the battery stickers to the battery and to the phone. The battery appeared to be aligned perfectly within the area to lay it in, but the connectors were not, apparently, perfectly aligned. The screw-in-place cover over the terminals appears to be holding the connection securely in place. Also, I had to use a hair dryer to free up the iPhone battery, as the pull strips kept snapping, even with very slow pulling as directed.
I have significant experience and comfort in working on Apple laptops, desktops, PCs over the years, including replacing hard drives and memory, and, thus, was willing to take on this iPhone repair. I would advise all to go slowly and carefully. I think the job earns every bit of being deemed "Moderate" in difficulty.