Painting a Canvas on a Wall

The artist takes a photo of the wall that he's going to paint and then uses Photoshop to layer his artwork over it. The good thing is that my work's quite forgiving, right? So if I do make a bit of a mess up, quite often nobody will notice. You know what I mean? I can just change it and it's okay. But also sometimes you're dealing with much, much bigger pieces. Like when it's outdoors, it's like the side of a building or something like that. And in that case, you have these other methods of scaling the image so that you take the image that's small on your phone and make it the size of the side of