By a few spools of ribbon in any color and pattern you wish. I would make sure the ribbon is at least 2 inches thick for more strength when the ribbons fill up. Cut the pieces to the length of the door plus a few extra inches on the top and bottom. Use duct tape, or something similar, to secure the ribbon to the back side of the door.
Last year I used plain clothespins, but I wanted to make them more festive this year. I bought three sheets of Christmas-looking scrapbook paper. I traced the clothespin on to the paper, cut them out and glued them down. That simple. Then I just hot glued a button to the top for a little extra detail.
How do you display your Christmas cards? Would love some new ideas!