It's not that complicted. We usually used it to communicate messages in movies. However it was used in personal communications as well.

You see when the key word, in our cases, the word ring or some form of this word, is mentioned, the code is activated. The first word after the ring is in code, as is every eleventh word thereafter. When the ring is said again, the code ends.

[He'll pass over a page that has an example of this. The words that are coded are underlined. It reads, this is an example of the Sebald Code, and it is inside a letter about an book]