I'm a do-it-yourselfer but have a lot of erperience with tile. Laying on the diagonal is not much more difficult than on the square. It needs to be a perfect 45 degrees. I spend a LOT of time in the layout stage. I lay our a line of tiles on the diagonal, account for the grout lines and see where it stops. Then make adjustments so that you don't have too many tiny pieces and cuts at one end. When I'm satisfied with the "primary" line I lay out a perpendicular line so I can check out those ends too. I make adjustments to both lines until I'm satisfied where all the tiles will be. Really important is the first "sight line" when you approach the room. Tiles on the diagonal can meake a smaller room look bigger. I just did a small toilet room off a master bathroom yesterday. I was a lot of work but visually very impressive. My biggest job was hekping friends don their entire first floor except the family room. The sight line went from the front door into the kitchen. It's spectacular! Hope this helps.