Things You'll Need
Caulk gun and caulk
When you're tiling two adjacent walls, an issue you'll need to deal with is what to do at the corners, where the walls meet. Since walls are tiled from the middle, building outward to the sides, and it's unlikely all the tiles on your wall just happen to fit perfectly; you will likely be dealing with cuts in that corner. The corner lines should be caulked rather than grouted, since walls move against each other over time, which will crack the grout.
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Hang all the full tiles over both walls leading to the corner, starting each wall at the middle of the bottom and building up and out, course by course. Set the tiles in thinset mortar, spread with a notched trowel, putting spacers between them. Stop each course when a full tile won't fit in the corner by the adjacent wall.
Measure from the edge of the last tile in a course, to the corner. Subtract 1/4 inch from the measurement you come up with, plus the width of a tile spacer. So, if your tile spacers are 1/8 inch, you will subtract 3/8 inch from the space. Cut a tile at that width on your tile cutter.
Apply mortar to the back of the tile with the notched trowel. Press the tile against the wall with the cut side facing the corner and the other edge next to the last full tile in the course. Space the two tiles consistent with the rest of the tiles (generally 1/8 inch), which will leave a 1/4-inch space at the corner.
Repeat the process for each corner tile up the wall, and up the adjacent wall, all with the cut sides facing each other in the corner.
Let all the tiles set for a day. Pull out the spacers.
Grout both walls, spreading the grout over the tiles with a grout float. Press the grout into the lines between the tiles, while scraping it off the surface, then wipe up the excess with a damp sponge. Don't grout the corner line; leave it open.
Allow the grout to set for 48 hours. Use your caulk gun to fill the corner line, running a line of caulk from top to bottom, filling the line completely. Let it set for 24 hours.