My battery would crash at 90% charge if I tried performing any sort of power-heavy action, such as taking a picture and sending it in an email. It would display the low battery warning when turning it back on was attempted. When plugged back in, the phone would be back up in a minute, showing a full charge.
I watched about 10 videos on how to do it, so it went pretty smoothly. The #3 ribbon didn't pose any problem since I was prepared going into the breakdown.
My battery was incredibly hard to separate from the case. It felt like I was going to bend the phone case trying to pry it off. I bent it a bit at the top corner, but after a while of poking and pulling, I managed to break the corner's seal; it all came easily after that.
The only issue I had with it all was the reassembly. Most guides and videos you see don't show it as much. I couldn't get my logic board to sit down as it should have on the left side. I may have had my battery slightly misaligned on the left side. After 5 minutes of screwing with it, I did what I shouldn't have done and started screwing it back down. It slightly "snapped" back into place at the point (the phone still works).
After around 25 minutes, I had a phone that feels like brand new, since I can use it as it was meant to be used years ago. I'm on it constantly, once again, as I have no fear of it dying away from a power source.
I recommend watching several different breakdown videos. You will hear different tidbits of advice from different people. Also, you will be aware of quirky difficulties, like the third ribbon connection.
Be patient with your old battery. It can be a bear to remove.
When aligning your battery, you may want to place the logic board back on top of it before you tape it down, to see how it will line up if it had been taped down.