Paint the pre-primed canvas cloth with your chosen base color of flat exterior paint. Let it dry. Straighten out all the edges with your hands.
Mark a line 1/2 inch from the edge all the way around with the yardstick and pencil.
Following this line, cut off the 1/2 inch of fabric.
Apply the fabric glue along the entire edge of the cut cloth to prevent fraying. Let the edges dry entirely. You can leave the cloth with this rough edge or turn the edge under, as in the next step.
Turn the cloth over to have the wrong side up. Fold the outside edge over 1-inch. Apply a heavy stream of fabric glue inside the folded edge. Mash the folded fabric and glue together as you go around the rug. Leave laying wrong side up until the glue has dried thoroughly. This creates a finished edge when it is turned back over.
Turn the rug right side up and put another coat of your base flat exterior paint on the rug. Let it dry.
Decide on the design you wish to paint on your canvas rug. You can hand-draw the outline of a picture or design, or tape the canvas rug to a wall and use an overhead projector, then just trace the design with the pencil. You can transfer larger designs using graphite paper. Or you can do straight-edge designs and use blue tape to mark off the areas. Press the edges of the blue tape down so paint won't ooze under it. That way, you will have clean straight edges.
Paint your picture or design as you wish, using brushes, stencils, rubber stamps or any combination of those, in acrylic or latex paint. Let it dry thoroughly. Then apply a coat of non-yellowing polyurethane to your finished rug.