Start by getting a " level line ". Say you have a 12" tile, messure up your 12" on each end of the wall and make a mark. Next snap a chalk line from mark to mark. Check that line with a level, if its not level raise one side of the marks until it is level.
After that its the same as on the floor. Spead an even coat not quite to your line. Be careful to put your first row of tiles straight with your line. Press firmly when appling your tile but try not to bump or move them once you have them set. Put in your spacers as you go, the tiles should be just tight enough to hold the spacers up. Make sure that you follow the directions on your tile addhessive and give it plenty of time to set up before you apply grout. Good luck with your project.
After that its the same as on the floor. Spead an even coat not quite to your line. Be careful to put your first row of tiles straight with your line. Press firmly when appling your tile but try not to bump or move them once you have them set. Put in your spacers as you go, the tiles should be just tight enough to hold the spacers up. Make sure that you follow the directions on your tile addhessive and give it plenty of time to set up before you apply grout. Good luck with your project.