Stand the canvas up on a table or other flat surface, positioning it right-side up, as it would hang on a wall.
Place your right finger on the spot on the painting where you'd like the light source to glow.
Place your left finger on the back of the canvas, finding the spot on the back that exactly matches your finger's position on the front. Mark the spot on the back of the canvas with a light pencil dot.
Evaluate the light source. The more precise the light source, the closer it should be to the canvas. If your artwork's depicted light source is a candle, your added light should be 1/4 inch from the canvas. If it's the sun, your light should be 1 inch away.
Cut the foam cylinder to one-half-inch shorter than the distance from the top edge of the canvas frame to the spot you marked.
Place the foam cylinder on the frame of the canvas on the back side, making sure it creates a straight vertical line from the canvas frame to the marked dot. Position the cylinder close to or far away from the canvas, per your evaluation in Step 4.
Use the hot glue gun to glue the foam cylinder to the frame of the canvas. Apply a generous amount of glue.
Allow the glue to dry.
Glue the faux candle to the foam cylinder, positioning the base of the candle on the base of the cylinder. The candle will be hanging upside-down.
Allow the canvas to dry overnight before hanging.