Start with a Lock & Lock ™ container or any water-tight container.

Caveat: Do not use dollar store containers. I have yet to see a watertight dollar store container. Do not use glass - broken glass happens.

I like to leave this note in the letterbox. Most of my letterboxes are also posted on geocaching.com. This note let's geocachers, who are unfamiliar with letterboxing, know that the stamp remains in the box and is not traded out. It also explains what to do with the stamp.

Click image for a larger picture.

Print a sheet of business card size instruction cards [doc]

Print a sheet of business card size instruction cards [pdf]

Make a quick logbook by folding and stapling a few sheets of medium-weight together (paper that is too thin will bleed through). More logbook examples. Click image for a larger picture.

Then put the logbook in a baggie to protect it.

Note #1(from AQ's glossary):
commando, going
The term 'going commando' has been adopted by some letterboxers to refer to a letterbox with no plastic baggies protecting the contents. Lock-n-Locks, most noteably, are legendary for their ability to repell water. However, a baggie that gets caught in the seal can let water get in, thus removal of the baggie and 'going commando.'

Note #2: if you 'go commando' make a felt pocket (I use no sew fusible tape) to place your stamp in, so that the ink from the stamp doesn't get all over the logbook and the rest of the box.

The all-important stamp. I like to use a fine-tip marker and write "Leave in box. Do not trade." on the back of the stamp and if there's room I'll add my email address. Example

If you wrap the stamp in felt or paper towel, rather than use a felt pocket, place it in a baggie. (I bought an assortment of small baggies at the dollar store). Click image for a larger picture.

Optional:
Add a pencil and a stamp pad. The stamp pad is optional and often not a good idea. If water gets into the container the ink could destroy the logbook. If you do not include an inkpad, as a courtesy you could write "bring your own ink" in the clues, but most letterboxers carry their own.

The pencil could be optional too but it's probably nice to include one.

All contents packaged and ready to hide.

To make the box easier to hide from view you could cover it in camouflage tape.