In order for the new adhesive to properly seal, you must remove all the old screen adhesive and its residue.

Be very careful not to damage any of the spring contacts around the perimeter of your phone during the next two steps.

Use the flat end of a spudger to scrape up and remove all the old screen adhesive from the perimeter of the frame.

If you're having trouble, apply a few drops of highly-concentrated (greater than 90%) isopropyl alcohol to the adhesive and scrape it up.