I had an old canvas that I had bought ages ago and wanted to use the structural part of it as a new frame for a picture I printed on the large format printer at Techshop Menlo Park.
Step 1: Take the canvas off the frame and mount the plywood
In order to get to the bare frame you have to remove the staples that are holding the canvas in place. I grabbed a screw driver and did just that by wedging the screwdriver under the staple and prying it up. Careful not to nick yourself with the screwdriver if you apply to much force and slip.
Once the canvas is off, the frame is exposed. I wanted the picture to sit a good distance off from the wall. To accomplish this I cut down a sheet of 3/4' plywood to brad to the top of the frame. This not only gave me the height I wanted but also supply a realativly smooth surface to adhere my picture to. To mount the plywood to the board I used a pneumatic brad gun and put several brads along the parimeter of the frame as well as down the middle.