Place your poster over the canvas. If your poster is larger or smaller than your canvas, decide how it should be centered before proceeding. Even with matching sizes the poster may need small trims to fit the canvas.
Brush a thin layer of acrylic gel medium onto the back of your poster. Acrylic gel medium is a clear gel used by artists to extend acrylic paint, achieve image transfers and create collages. It's safe to use indoors. In this case, use it as a decoupage glue.
Brush a layer of gel medium onto your canvas. Obtain even, full coverage. As you brush, overlap the edges of your canvas to ensure that the poster edges will be glued securely.
Place the top edge of your poster along the top edge of your canvas. Press down gently. Continue applying pressure between poster and canvas, working your way from top to bottom.
Use a rolling pin to press out any wrinkles or air bubbles. Work from the center of the poster outward.
Let the poster dry completely before trimming any excess paper with a craft knife.