Measure the width of each side of your corner tile, if you're using a specialty tile. Add the width of your grout line to each measurement. Mark the wall in the corresponding spots to the measurements. Use a pencil or chalk and a straightedge or a ruler to draw a line up each wall or across the surface. This marks the edge of where your corner tile will be set.
Mix a batch of thinset mortar per the manufacturer's instructions. If you are installing specialty corner tiles and have mortar left over from setting the surface or wall, use it.
Use a notched trowel to apply a thin layer of thinset cement mortar to the corner of the wall or surface.
Place a spacer between the floor and the first tile, if needed. Set the first tile into the mortar. Press down lightly but firmly to set the tile in place.
Use masking tape, if needed, to hold the tile in place while it dries.
Hold a straightedge or level to the top of the first tile to make sure it's plumb, even and straight with the other tiles. Unless uneven grout lines are part of the design element, you want the grout lines for your corner tiles to match up with the tile on your walls or countertop.
Put a spacer on top of the bottom tile. Set the next tile. Tape it in place with masking tape, if necessary. Repeat the process until you've covered the entire corner. Let it dry and harden overnight.
Remove any masking tape from the surface of the tiles if used. Mix the grout per the manufacturer's instructions. Pull a grout float and grout diagonally across the entire surface of the wall or counter to force grout in between the tiles. Scrape off the excess from the top of the tiles with the float as you work.
Wipe away any excess grout on the surface with a damp sponge. Rub the surface with a soft cloth or rag to remove any haze from the surface after the grout dries.
Use a tile sealer to seal the entire surface.