Usually start from the floor - if the floor is uneven start from the top left on the ceiling. Ceiling should be garaunteed level as the joists have to be level for the roof to sit correctly.
From top left, tile down one row vertically until you cannot put one full last tile. Then put a batten (long piece of straight wood) down across and measure from either side and the middle from ceiling to batten (should all be equal if level). Wait to set (24hrs) and finish all the tiles above the batten. Then do the off cuts below the batten.