If using rubber cement:
First, dab or brush rubber cement on the back of the invitation, keeping at least 1/4 inch away from the edges. You don't want to add too much rubber cement so that it leaks out of the sides when you push the invitation down on the mat. But you also don't want too little on the invitation either– this makes it dry faster, which makes the two papers adhere to each other faster, which is not what you want because the last step is to slide the invitation around until you have it centered. When you have it centered, press down and let dry.
If using a glue stick:
I would recommend rubbing the glue stick on the back of the invitation and then placing the invitation onto the mat. You will have to slide/glide it around a bit to make sure it's centered. When you have it centered, press down and let dry.