Take your scrapbooking paper (or anything patterned) and cut it so it is just smaller than the card. It is easier to do this with a guillotine, but I just use scissors. Once its the right size stick it down with the glue
You might find that the paper isn't sticking very well, so use some hair grips to stick it down at the edges.
Now add the message piece with the glue in the same way, making sure it sticks down and hey presto a great message card