Replacing slate tiles on the side of a dormer window

Hi

I need to reroof the side of my dormer window ( similar to the picture attached ). I am planning to use reclaimed slate to match the other side.

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I am having a bit of an issue working out how to deal the with the angle of the roof.

What is the best practice for doing this ? I have started from the bottom and nailed a row working from the outer edge to the angle of the roof, cutting the tile which meets the roof. When I nail the next row, I am having issues keeping the level of the 2nd row when I meet the angle of the roof ( as it is now 2 tiles overlapped ). I can see this getting more akward the more rows I overlap above. Am I missing something obvious ?

Can someone please provide some guidance on the approach to this.

Hope this makes sense. Thanks in advance

Use a spirit level to either mark out head or tail lines.

Sorry maybe I wasnt clear with my question. I understand I can use a Spirit level to keep the line of the bottom of the tiles. The issue I am having is how to deal with the nailing the tiles in at the angle. I have done two rows which is fine but the next row up when I get to the angle of the roof, my last tile is sitting flush with the wall, whereas the row of tiles below have been built up.

Should I be bulking this out as I go ( working from the outer edge in ) or should I run a of cut tiles up the angle of the roof ?