If the tiles are smooth on the back, and the walls are quite flat, you can even use a 1/8" 'U shaped' trowel. This might help keep the morter from squishing out between the tiles as you set them. As always, put up a couple of tiles and then remove them to check for a good bond. If you are using thinset, mix it a little on the "loose" side....not runny, but loose. It should hold a ridge...but just barely.
Avanti Tile & Stone
"They Didn't Want It Good...They Wanted it Wednesday..."