Raster engrave the phone outline and Christmas tree on to the steel.
Use a jeweler’s saw to cut around the inner part of the phone outline. To cut out the hole, drill through the center, lace your saw blade through, and cut.
Cut the suction cup tape into strips and apply them adhesive side down on the back of the steel; about 5 thin strips evenly spaced should be enough. Peel the plastic masking off and apply to your phone. The backer will be removable and won’t leave a residue.