Thanks, Derek! It's actually the same process as this: https://steemit.com/photofeed/@johndykstra/the-process-of-building-a-perspective-illusion

I draw it on paper first, draw a sort of graph onto the set in chalk, take a picture, upload the drawing and layer it above a photograph of a scene, see where the drawing lines up on the graph, which tells me where to draw my outline on the set!