Normally, the backsplash behind your sink sits flat on the wall. But in some kitchen configurations, the sink is tucked in an inside corner, meaning your backsplash needs to traverse the corner. It's not difficult to tile a corner backsplash, but it is something that has to be handled correctly to prevent the line from looking out of place, and to assure that water doesn't get under the tiles in the corner, which is the most vulnerable part of any tile wall in terms of moisture breach.
Use your tape measure, level and pencil to mark out the area for your backsplash. Make the size of the area the same on both sides of the corner. If you can, make both sides wide enough to use entirely full tiles, with no cuts. If not, make sure that the end-cuts will be approximately half a tile, not significantly more or less. If the backsplash is going to be under a cabinet unit, plan on taking the tile all the way up to the bottom of the cabinets; otherwise, make the height 16 to 18 inches.
Use your adhesive trowel to spread adhesive over the entire section, starting on one side of the corner.
Press your tiles into place, starting at the bottom outside edge of the section. Affix all your full files, putting tile spacers between them as you go.
At the inside corner, if cuts are necessary, make them on your tile cutter. Measure the cuts so the cut side of the tiles butt right up against the adjacent wall. Hang the tiles.
Repeat the process on the other side of the corner. This time, make the cuts at the corner so that the cut side of the tiles are about 1/8 inch from the face of the tiles on the other wall. Let all the tiles set overnight.
Grout the tiles on both walls, pressing your grout into the lines between the tiles with your grout trowel. Don't grout the corner line between the two walls, or the joint between the backsplash and the countertop. Wipe off the excess grout with a damp sponge. Let the grout set for two days.
Use grout sealer to seal the grout lines. Fill the space in the corner with a bead of caulk. Do the same for the line between the backsplash and the countertop.