Excellent Tools and Video/Pictorial Tutorials
My 6s Plus battery was not lasting 24 hours, even with minimal usage. Plus, the story that Apple was intentionally slowing down my phone after updating to the newest iOS was troubling. I wasn't willing to wait 6 weeks for Apple to replace our batteries (I replaced my wife's 6s battery as well), and iFixIt's DIY price was the same.
Smooth and easy. The magnetic screwdriver bits REALLY helped while removing each super-small internal Philips screw. I took each step slowly, using a magnetic cup to hold all the itty bitty screws. With patience, I was able to cleanly remove the screen water-seal gasket.
The suction cup step requires extreme patience - especially at the very beginning. It did not open up immediately as the video tutorial showed. I had to apply pull-tension and simply hold it, watching while the screen seal adhesive very slowly let go. Then, I would release the tension, adjust my grip on the suction cup and my phone, and re-apply pull tension. I had to repeat this 'pull and watch' step 4 or 5 times. Eventually, the gap was big enough to slide in the opening tool.