It lines up with the hole in the bottom of the phone. That's the long rectangular rubber part. The round part actually pressure fits over a round thing near the bottom of the motherboard. Once you find it it will make sense because it will fit perfectly around the round thing and extend exactly to where the hole is on the bottom of the housing.
It's a pain in the butt to get back on, especially when I reassemble the phone and I look down and see it sitting in my parts dish still, I'm like FUUUUUU-