1. Get a small notebook. It is suggested you use a small sketch pad but we used a spiral bound set of 3x5 ruled index cards that we are using from back to front so as not to be bothered by the lines.
2. Pick a rubber stamp that will represent your family or group and use it to decorate the front (in our case back) of your notebook. I suppose technically you don't need to decorate your book but even in my uncraftiness it felt like I should.
3. Pick a name to represent your family or group.
4. Pull together a few necessities: your chosen stamp, ink pad, pen, wet wipes (to clean your stamp), and a compass (you won't always need the compass). We put ours in a Ziploc bag for easy storage and easy carrying.
5. Go here to search for a letterbox.
6. Follow the clues or directions given until you find the letterbox. (Be aware that occasionally the letterbox will be missing. Many of the pages giving the clues will let you know if it has been reported missing or will let you know when it was last found but not all will. My advice is to either pick a couple backup letterboxes to search for or choose a location/hike that you will enjoy regardless of whether you find the letterbox or not.)
7. Once you find the letterbox and retrieve it (without drawing the attention of any other hikers or individuals in the area), make your mark with your stamp in the notepad that has been left in the box and then using the stamp from the box place the stamp in your own notebook. Then fill out any other pertinent information such as the date and your group's name.
8. After you are finished with the letterbox replace all the items and put it back in the exact spot you found it (using stealth so as not to draw the attention of any people passing by).
9. After you return home go back to the page you got the clue and let the letterboxer know when you found their letterbox (also let them know if you didn't find it or if there was a problem with the box--a missing stamp, water damage, etc.)
Go here for more information on letterboxing (there are some short articles on getting started that I found very useful). This is the best link at that site for searching for a letterbox (or at least I think it is).
This is the first one that we tried (and didn't find).
This is the first one we found.
If you live in Salt Lake, Utah or Wasatch Counties here is a good blog with recent letterbox clues. (Use the sidebar to find the clues)
If you live in Boston I want you to try to find this one. You could pretend to be James Bond or Jason Bourne. Just don't get caught!