Girlfriend's iPhone 6's battery has been acting very erratically. It would go from 100% to 20%, and sometimes it would just shut down. Once it has shut down, it then asked you to connect to the charger to power it on. I figured it would be cheaper to fix it myself than send it to Apple for a $80 replacement battery.
Everything went smoothly until the part where I had to remove the adhesive strip off the battery. That was a bit challenging because I tore the strip apart, but I happened to have a very fine tweezer to pull the strip off so I could pull it by hands, again.
Reversed the order of the instruction to put everything back the way it was, and the phone powered on without issues. Phone has been running for 3 days, and all the battery issues are gone.
It is recommended but not necessary to have a magnetic bowl —like the one you can get from Home Depot—to insure you don't lose these tiny little screws.
Also, when prying up the screen/digitizer off the phone, don't put too much force at one single point, it might slightly bend the bottom of the screen.
P.S. I wish the replacement battery has a label that shows the manufacture date like the Apple OEM ones.