ok..progress, I thought about just doing it free hand, but thought cause of the size I might be better off starting with a proper design. So I have a picture, and I've gridded it. But now I'm thinking how to grid the wall, could just pencil a grid on, but will be a PITA if i have to erase the lines, or will the paint diffidently cover it? My other option is to bluetac string, which might take some more time, but might actually be easier than drawing straight lines on a wall.

any ideas? Thanks everyone!