First, take your wood panel and lay it on top of your canvas. Using a pencil, trace about 1/8-1/4 extra all the way around. It's important to have a little bit of extra wiggle-room.
Using a junk brush, apply some glue to the panel. There's a magic amount of glue to use, and it will take some trial and error before you find it. Too much glue, and it will ooze out the sides. Too little glue, and your canvas won't stick down.
With the primed side of the canvas facing down, place the glue-side of the board directly onto the canvas. Press down firmly, and then flip the panel over.
Using a soft rubber brayer, roll across the entire panel, working from the center on outwards. Once you are satisfied with the adhesion, place a heavy book on top of the panel and leave to dry.
After the panel is dry (a few hours), carefully run a razor blade around the edge.
And there we have it, inexpensive panels ready for painting!