Battery on my 6s was jump several percentages at at time when it got below 40% or would drain a few percentages a minute when just viewing the phone not really using it.
The kit comes with all the tools needed as well as the adhesive for the battery and screen. I didn't know it came with the different adhesives so I bought some off of Amazon. The battery adhesives went fine but the screen adhesive, I messed up on starting to go up the sides of the screen and when trying to pull it back off, it stretched out...ruined. Luckily I had the backup and was able to complete the screen adhesive. Also, the old screen adhesive took a little bit of time to get off of the display. Put all back together and my phone battery is awesome!!! And I didn't have to wait on Apple to do it in store or by mail. I'm technically inclined so I liked the challenge.
Make sure you get all of the old screen adhesive off; take your time with the new screen adhesive (or don't use one at all but that would decrease the water resistance of the device).