The backing is stable, rigid, and clean
The backing, or substrate, might be concrete, plywood, drywall, or cementitious backer board. No matter the material, it must be flat and solid; any warps, bumps, or springiness can cause tiles to crack. The surface must be free of oil, grease, dirt, paint, and old grout or adhesives.
The pattern avoids awkward cuts
Measure the number of tiles needed, horizontally and vertically, to reach the ends of the walls, keeping joints uniform. Shift the pattern left, right, up, or down so that there are no thin slivers or small pieces of tile in highly visible areas. Use corner trim or tile with bullnose edges to avoid exposed edges at the ends of runs.
The tiles sit straight and flat
Lay tiles using a horizontal level line and a vertical plumb line as guides. No individual tiles or corners should protrude from the surface. The field should be finished with mold-inhibiting caulk wherever tile meets adjacent surfaces, such as walls, tubs, counters, or cabinets.
The finished walls get regular upkeep
Most glazed ceramic tile needs little more than cleaning with a mild, nonabrasive soap. But some crackle finishes and most unglazed ceramic tile, such as terra-cotta, need sealing to keep out water and dirt, as do cement-based grouts. Follow the manufacturer's directions for recommended sealers and frequency of application. Replace caulk when it starts looking moldy or grubby.