This is how I suggest (and did) our kitchen backsplash.

This is backsplash tutorial

Prep: If you are using outlet extenders put them on prior to beginning work. I suggest using screw extenders. Verify the counter is level Mark a spot an inch or 2 from the top of the counter at the far ends. Put a couple of nails in and use a chalk line to mark a reference line across the length of the wall. If the counter is an L or U shape make the reference around the entire congruent area.
From here determine the lowest spot that is the starting point which will mean you might have to shave every tile on the bottom row.
lightly sand the wall with 80 grit paper, remove any major chunks from the wall with a trowel.

Mounting: Mastic has better hold than mortar and does not require mixing Work from the bottom up use a v notch trowl and cover area thoroughly. Pull the trowel sideways to help the tiles from sliding. Use spacers between tiles. make sure if the wall has a wave to build the mortar up enough to make it level. put tile up and press firmly in place Let dry for full 24 hrs

Grout: use a grout that does not change color when wet. This can be an epoxy grout, which is a little more difficult to work with and hardens in about 20 mins. can also be an additive to normal grout. if using sand grout, use a thinset sealant over it to seal trowel the grout on and sponge off examine the wall by looking straight on and from the side(s) to see if light reflections show imperfections or dark areas.

Seal bottom with silicone