Piranesi uses an export of your 3D application. So although Piranesi is a 2D pixel based application, every pixel knows where it is in 3D space!
This is why Piranesi knows what the perspective is and scales cutouts and textures exactly the right size.
You need an EPIX file that can be exported out of almost every application:
Now open this file in Piranesi and you can start applying textures, backgrounds, edges, paper styles and light effects right on the image. Because Piranesi knows where pixels are in 3D space, you can lock editing to certain (parallel) faces, grounds and backgrounds:
The interface looks very familiar to other applications so you can quickly adapt to it. Once happy with all applied textures and colors you can start applying Styles. Anything is possible, from charcoal sketch to water-colour painting!