We present an overview of instructions for the use of European historical cipher keys in early modern times. We describe the structure of instructions and the content presented to the key users. We exemplify various key instruction types and give a text edition of typical examples in various languages. The study is based on the analysis of more than 1,600 cipher keys collected from archives and libraries in ten European countries. We examine the practical implementation of cipher keys to the extent that instructions offer insights into everyday cryptographic practices. We focus on the typical rules scribes were expected to adhere to and the common errors they were instructed to avoid. We aim to reconstruct the apprehensions and considerations of the authors of cipher keys: They sought to offer assistance to users while likely harboring concerns regarding the potential misuse of their intellectual product. Given the secretive nature of cryptology, the documentation of knowledge transfer is scarce. In addition to the detailed manuals authored by well-known cryptologists, anonymous cipher key instructions offer valuable insights into this knowledge transfer process. By studying these instructions, historians gain direct access to a realm of knowledge that would otherwise remain hidden from their view.