Any leftover residue will compromise your iPhone's seal. Be sure to clean the frame thoroughly.

Wrap a microfiber or lint-free cloth over the point of a spudger and apply a few drops of high-concentration isopropyl alcohol (over 90%) to the cloth.

Wipe in one direction along the perimeter of the frame to clean the adhesive residue.

Wait a few minutes before proceeding to make sure any excess alcohol has evaporated.