Lay the poster on one of the pieces of cardboard.
Measure the distance between the edges of the poster and the edges of the cardboard to center the poster on the cardboard. If the poster is centered, all of the measurements are the same, assuming you chose a piece of cardboard 3 inches bigger than the poster on all sides.
Trace around the poster with a pencil. This creates a square drawing on the cardboard.
Use a box or craft knife to cut out the square.
Lay the poster on the cardboard so the image on the poster is centered in the square hole you just created.
Put a thin bead of glue around the edge of the cardboard that extends past the poster.
Lay the other cardboard on top, sandwiching the poster between the two pieces of cardboard.
Let the glue dry before displaying the poster.