Please excuse my discolored photos, I was doing this at night.
After they match up correctly I will make a mark with a pencil in the middle of the card so it doesn't show once I'm done. By making a mark you can then remove the top die-cut and place your tape or glue on your bottom piece in the the right place and position them back together easily by lining up the edge with your mark.
Don't you hate it when you get something in the wrong position and it's stuck there forever? I do!
I made the line large and dark so you can see it. All you need is a dot.