Tiling a roof

No you can't just jump into Tiling a Roof. It has to be planned out meticulously.
Have you spaced your Battens out correctly to match the Tile sets.
You will need all the relevant measurements Width including overhang. length of roof front back. pitch angle. you will also need tile Manufacturers instructions.
I prefer to get the center line of the Roof and use that as the starting point.
Use the center line and set out tiles either side of the line until you reach the edge. You should try to finish with full and half tiles or as near as possible.
You might even set the center of a tile right on the center of the line.
Make sure the first row goes up square to the Facia and the side of the building otherwise it will go off sq very quickly.
You also need to keep perpendicular lines to check against as you go, So as you don't have too much tiles fixed and then find out your gone off line.

If you take the measurements above to the tile Manufacturers they may set it out for you on paper.