On an old 5S, the metal retainer bracket snapped in easily. There was no sliding, the rear edge dropped in straight, it hinged down, and locked almost before I knew it. The lack of a click or any positive feel on this critical connector surprised me a lot. I had to eyeball it with a magnifier to feel sure the ribbon connector was really in its socket. Works perfectly after the replacement, though.
Leave an equal sliver of empty space on all 4 sides of the battery. My original installation had this. I suspect it helps reduce transfer of vibration and shock from the battery to the rest of the internals.
I used the edge of a desk as an alignment guide. I would *not* eyeball this alignment, unless you don't mind wasting your first pair of adhesive strips.