I'm about to announce the Hufflepuff Wizard Cracker Exchange, so here's a short tutorial on how to make your own Cracker.
If this were a real Wizard Cracker, the lucky recipient would pull on the ends and it would explode with a bang, revealing all sorts of goodies. Since mailing something that explodes is probably a Really Bad Idea, we're going to go for the look - and the goodies.
Supplies: You'll need a cardboard tube, some tissue paper, tape (or glue), scissors, goodies to put in the cracker, and stuff to decorate the cracker. The dog is optional and not recommended, but my EvenWorseDog really likes to steal and chew up paper towel tubes.
Step 1: Cut the tube to the length needed for your goodies, and roll it up in tissue paper that's a few inches wider than your tube is long.
Step 2: Twist the paper on one end of the tube to close it, and fill it with goodies through the other end.
Step 3: With both ends twisted shut and tucked in, pick your paper for the outside of the cracker. I've had to tape two 12" sheets of scrapbook paper together here, but the seam will be hidden by my decorations. You want something that will wrap around the tube with a bit of an overlap, and leave 4-5" at each end of the tube.
Step 4: The decorative paper is taped around the outside of the tube. Pinch the ends shut - you'll want them to be fairly snug.
Step 5: Fasten the ends shut with yarn, ribbon, or tape, and decorate as you see fit. Your cracker is ready to mail!
Possible embellishments - you could always add some curling ribbon, to simulate an explosion. Or confetti - if you warn the recipient to be careful about opening it. Here are some examples of Muggle Crackers, for inspiration.