Hold the front piece and back piece up against each other to ensure they are the same size, and if necessary use scissors to trim any areas that don't overlap.
Then rub glue on the back of the card stock, and place the front piece previously made against it. Gently press to ensure the two pieces are in full contact. Obviously, you'll want to make sure the front and back pieces are oriented correctly so that one isn't upside down after you glue them together.
Consider using PYMII (Preserve Your Memories) to cover the front and back to help protect the paper, but this isn't a requirement since the paper will be covered by glass. If you do use PYMII, make sure it is totally dry before continuing.
