Tiling a shower protects the walls from moisture. One of the more complicated aspects of this job is how to handle the inside corners. The walls should be tiled working from the middle out so that the spaces along each interior end of the tiling is similar. Your cut tiles there must allow space between the adjacent tiles and the corners. Caulk instead of grout is used in the corners because they are especially vulnerable to moisture breaches.
Hang tiles over the wall's surface. Stop tiling when full tiles no longer fit into the corners where the walls meet. You will be left with thin spaces running up along each corner. Allow the full tiles to set overnight.
Measure the space between the edge of the full tiles and the corner. Subtract 3mm (1/8 of an inch) to allow for a standard grout line between the tiles. Subtract another 6mm (1/4 of an inch) to space the tile from the wall. So, for example, if the space by the wall is 102 mm (4 inches), your final measurement is 93mm (3-5/8ths inches).
Cut a tile to the right width using a tile cutter. Spread thin set mortar on the back of the tile with a tiling trowel. The mortar should be about 3mm (1/8 of an inch) deep on the tile.
Press the tile to the wall, with the factory edge facing the full tile and the cut edge facing the corner. There should be 3mm (1/8 of an inch) of space between the two tiles and 6mm (1/4 of an inch) of space between the cut edge of the tile and the wall.
Do the same thing at the end of each row of tiles on the walls, piecing them in individually. Measure each space separately because there might be minor variations even if all the spaces along one side of the corner appear the same.
Allow the tiles to set for a day. Use a grout float to spread grout over the walls, pressing it into all spaces except those in the corners. Use a damp sponge to wipe off excess grout.
Allow the grout to set for a day. Caulk the corners, filling the spaces there with smooth beads from top to bottom. Allow the caulk to set for a day.