To tile the underside of a door or window opening in a wall, cut a thin strip of 6mm ply the same width as the opening and three lengths of timber 6mm less than the height of the opening to act as props for the strip. Size the area to be tiled, as full-width tiles may not fit the opening. If it is necessary to cut the tiles, either use a trimmed tile in the centre or have odd tiles at each end to achieve a balance. Always use the second method if the tiles have to fit in with an existing tiled pattern on the wall. Use round-edge tiles, with the round edges facing outwards.
Start by spreading adhesive along the area to be tiled and then press the first tile into position. Put the strip in place under this tile and support it with one of the vertical props. Taking care not to dislodge the prop, spring the strip down and place the second tile in position. Put the second prop under it. Pull the strip back again and insert a third tile, this time moving the second prop along under the third tile.
Continue the process until either the middle or the end of the reveal is reached, depending on the starting point. Then, keeping the second prop at this point. Start working with the third. Move this prop along under each tile until the tiling has been completed. Leave the strip and its supports in position until the adhesive has thoroughly dried. Then grout the joints.