Identify your slang. Pick a specific type of slang and time period. Being exact in the planning and research stage will help make the task of compiling the book a lot easier. A good example would be "hip-hop slang from the early 1990s." Check to see if someone else has already published a book on the same topic. If so, you may have to alter your scope.
Acquire a word list. This is the most difficult part, so take a systematic approach. Start by examining the pop cultural aspects that drive a particular type of slang. Study the language used in relevant music, films, videos, books and articles.
Interview people. Track down a few master speakers and spend some time with them. Record their speech patterns. It may be helpful to put them in a group so you can observe how they speak with one another.
Compress and alphabetize the word list. Cross-reference all the items to keep the language consistent. Eliminate unauthentic or outdated words and phrases. Show your first draft to a master speaker for further edits.
Write an introduction. Since this not a regular dictionary, you must provide adequate context for the reader. Elaborate on the history of your chosen slang. Identify its main driving forces and explain how you put together the list of words and phrases.