The instructions make it clear what to do. But, if this is your first time to do this then I strongly recommend reading the comments below placed by others before you start on this project. Review the instructions several times before starting and have them handy as you do it. Take your time.
I found it easier to connect the battery and then set it into place, else you can find yourself fighting with the the battery connector. Not every phone battery is glued in place exactly the same. Wife’s phone battery did not want to come out, fought me the whole way. My phone battery popped out easily.
Also, check the tool kit before you start. I had fortunately ordered two batteries and two Fix-It kits - good thing - because one kit had three Y000 bits and no P2 Pentalobe driver.