Conversion to process rules is included for existing customers and customers who started out not using process rules and now want to transition to them.
New customers can skip this information and follow the instructions in Preparing for build management and Following the build process. Process rule usage is described for new users in those topics.
The build management methodology uses purposes, process rules, and folder templates. All documented methodology discussions are centered around these features.
A site can convert to process rule use at any time. Converting at the start of a release is easiest because you can ensure that the team is using process rules consistently.