The bulk of the bed is made from an old vintage chair. I first disassembled the chair, removing the back and front. Next I sawed the back legs off. The front legs were left attached to the original cross piece. The sides of the seat were discarded.
Next I built a box using 1x4 lumber, and to this box I attached the chair's back and the chair's front legs, which I inverted. The size of the box should be dictated by the size of the dog!
Inside the box I fastened some 1x2 rails (see the 2nd photo) which are used to support the mattress portion of the bed. I cut a piece of plywood to fit the box, added some foam rubber, and upholstered it. The mattress simply sets inside the box -- I didn't fasten it in place so it can be easily removed later to clean the upholstery.
The last thing I did was paint everything, and add some old brass doorknobs as decorations.
This is a relatively easy project and requires an old vintage chair, a few screws, some scrap lumber, foam rubber, upholstery, paint, and a couple of old doorknobs.