Start with a clean dry water bottle, peel the label off. Have the munchkins write their letters, or draw pictures and then wrap it up in a tight scroll (small enough to fit through the top of the bottle). I taped it then tied it to secure it:
Then I used a touch of elmers glue to the inside of the cap to seal it, and then as an extra measure- I taped it.
Write out your address and adhere it to the front of the bottle- you can put it in a ziploc bag (to protect it from the elements) and then adhere it,or you can tape over it with a clear tape (Clear packing tape works great, but I just ran out of it, so I used scotch and went around the bottle with it)
(please excuse the edit- I wasn't putting my real label up!)
Take it to the post office and have them weigh it for you, and send it on it's way!