Measure the square footage of the room with a measuring tape. Multiply the length by the width in feet to get the square footage of tile you will need to buy.
Remove ceiling light-fixture shades. Using the length measurement, make a small pencil mark at the halfway point on both sides of the room. With a partner holding one end, pull the chalk line tight between the two marks at the top of the wall against the ceiling. Snap it to make a chalk mark through the center of the room.
Brush ceramic tile adhesive on the back of each tile. To cover deeply textured popcorn ceilings, apply the adhesive with a putty knife. Begin in the center of the room; set the edge of the tile against the chalk line mark and press firmly. Install the first row along the chalk line mark and work outward toward the wall.
Measure the gap between the last row of tile before the wall and the wall itself. Measure tiles to fit, and cut them with a sharp utility knife and a metal straight edge or yardstick.