The usual way to start is by clicking on the Letterboxes menu. This will take you to a page with two parts. The top section allows you to use location based search and the bottom section allows you to search by a combination of specific criteria, e.g. letterbox name, city, etc. Fair warning that the search by location is a bit broken on the current server, but is working on the test server, www.letterboxing.co.
Each section has optional check box to include archived boxes. These are boxes that may not be available for a variety of reasons and are usually hidden from search results. See "Why are some clues archived?"