I bought a mirrored medicine cabinet for $13 from Home Depot (on the left), but I wanted it to look like a painting on the wall.
The picture frame on the right I brought years back from Romania but was way smaller than the medicine cabinet door.  
I also had an oil painting but the canvas was even smaller than the frame.  
I cut a few strips of 1/4" thick plywood as to make a frame the same size as the medicine cabinet door.
I first applied carpenter's glue on the back of the picture frame and I also nailed the plywood strips with the gun.
The back of the frame is now exactly the same size as the medicine cabinet door.
The frame so far.
I had these leftover polyurethane mouldings. It occurred to me to nail a frame around the plywood.
On the left, the frame so far. I now had to make the canvas fit the frame. I cut a solid piece of plywood as to fit into the frame. I planned to glue my canvas on this piece of
plywood. I also cut a frame of plywood of the same outer dimensions. On this frame I was planning to do a crackle finish and to put it on top of the canvas.
Once I glued/nailed everything, I caulked all the seams. I then painted the entire outer frame solid gold. Once dry, I glazed it with dark brown.
On the inner frame I applied a coat of dark brown. Once dry I applied a thin coat of beige crackle paint. Once dry, I antiqued the crackle with a tiny bit of gold.
I also antiqued the dark brown on the oil painting with white to better blend with the crackle frame.
I removed the mirror from the medicine cabined door and I glued/screwed the painting on the door.
I made a cut in the wall the size of the medicine cabinet. I framed the opening on the inside with wood studs so I could screw the cabinet onto the studs.