Carefully connect the display cable to the connector on the phone circuit board. All you should need to do is line it up and gently push it down until it seats in the connector.

Make sure the other connectors that are normally hidden behind the plastic piece are still connected properly, then reconnect the battery connector and reinstall the plastic piece. Make sure that the plastic piece is clipped into place properly before reinstalling the 4 screws.

Before reattaching the back cover, it's a good idea to turn on the phone one more time and make sure everything works, including the screen, speakers, microphone, cameras, WiFi, and cellular data.

Make sure the cameras are free of any dust, etc. before reinstalling the back cover. If they did pick up some dust, you can carefully brush it out with a soft paintbrush. Similarly, make sure the part of the back that covers the camera lenses is free of dust.

If the back cover adhesive didn't get too damaged in the removal process you can press the cover back on.

In my case, the adhesive came apart and balled up in a few places, so I ended up removing it and using more of the rubber adhesive I used for the edges of the screen. Since I always keep my phone in a case, I used very little adhesive to hold the cover on.

At this point you're all done.