Off the top of my head.
(Bearing in mind exchange of encrypted data is always vulnerable to the initial key exchange)
1) Choose a book*, obviously in digital form
2) Parse the book into a database. You need to be able to search for words and letters.
3) Encrypt your plaintext by matching words from your book, and noting the position. Obviously you will have many exemplars for common words, so your algorithm should try and use different word positions for repeated words (e.g. "The"). For words not in the book (technical, foreign) you escape and insert letter positions (again mixing them up).
4) You now have a text document which is meaningless without knowing which book was used. (And - see Sherlock Holmes - which edition).
Now you could just generate your own hash table. But then you'd have to distribute it. With the book system, you can distribute the DETAILS of the book by a separate, trusted channel. Say a face to face meeting.
However, my understanding is that the spooks are less interested in the contents of messages, as the sender and recipient relationship. In which case, posting to newsgroups would avoid the link.
*Magazine, newspaper, research paper.