Joiners is typically created by taking multiple photos of one view, and joining them together to give a wider view of the scene. David Hockney did this with his work in the early 1980s.

I took a photo from one of my shoots on Romanticism, and edited it to look like a joiner on photoshop. I selected an area and created a copy. Then I would change the opacity, exposure, or hue to make it stand out. I also moved the copies around the image to show a clear difference between them and the original photo.