Remove the Lightning connector assembly.

Use a plastic tool to scour any bits of adhesive residue from the rear case.

You can clean any remaining residue from the surface with isopropyl alcohol (90% concentration or greater) and a lint-free cloth. Wipe in one direction only, not back and forth.

Make sure the Lightning connector assembly is correctly positioned so that the two white dots on the iPhone's rear case show through the two circular cutouts in the Lightning flex cable. If they don't, the flex cable will remain misaligned and you won't be able to reconnect it to its socket on the logic board.