1. Slope the ceiling slightly. If you don’t, any steam that condenses there will drip down onto your shoulders.
  2. Protect wood framing with sheets of 6-mil plastic stapled over studs and joists. Overlap all edges by at least a foot.
  3. Seal the seams between the concrete backer-board panels (which are unaffected by moisture) with mesh tape and thinset.
  4. Cover every inch of plastered area with a waterproof membrane. Use either two coats of a roll-on liquid polymer such which dries into a seamless, rubbery skin, or embed sheets of flocked polyethylene.
  5. Tile and grout as usual. If using stone, apply an impregnator to seal the stone. You’ll need to repeat this treatment every two years.
  6. Make sure you seal around any fixtures that penetrate the tile—including shower and steam heads, digital controls, and valves.