Internal corners are the most common obstacle when tiling, quite simply because every room has them. If tile is being laid on more than one wall in a room, you will eventually have to deal with tiling a corner. There are a few simple rules to follow that will make tiling an internal corner very simple.
Measure the distance between the edge of the wall and the corner junction. Take 1/8-inch off of the measurement to account for grout.
Mark the measurement on a full tile using a fibre-tip pen. Score along the marked line with a tile cutter to cut into the surface of the tile, leaving a clearly defined scratch.
Place the scored tile on two matchsticks so that the cut line is positioned on the centre of each matchstick. Apply even downward pressure on each side of the scored line in order to snap the tile along the cut line.
Spread the adhesive or thinset on the back of the cut tile.
Place the tile on the wall with the factory edge of the tile next to the column of full tiles and the cut edge running along the corner junction. Continue up the corner junction, filling the gap with more cut tiles.
Repeat the same procedure for the adjacent wall. It is important to cut these tiles as neatly as possible and remove any rough edges with a tile file since the cut edge of these tiles will overlap the column of tiles on the first wall.
Check that the tiles are in the correct position by holding a level across the corner junction. Position the edge of the level so that one end is at the junction between two tiles on one wall, while the other end of the level is positioned on the corresponding row of tiles on the adjacent wall.
Position spacers along the corner so that they span the junction and maintain the necessary gap between, and along, the two columns of cut tiles.
If a tiling plan has been worked out correctly, you will usually require just under or just over half of a tile to fill the gaps between the last full tile and the corner junction.