We had a painted dry wall that I sanded to prep for the mastic. thin set was then applied at the back of the glass tile sheet, set it right at the middle fanning out through the end. Gave each tile sheet a little wiggle to push out air. I used black grout after an overnight wait for the adhesive to dry. The cleaning part, going after the black grout took most of my time. Electrical outlets were then changed to match the grout color.