Write up that stuff on a file and tell the agent to look at it. Say “take a look at file A as an example of how we do this sort of thing.” Good comments in the code help that explain how it works and what that patterns are are helpful, but you don’t need to go line by line duplicating the code.
I've found that this works for smaller projects but starts breaking down when you're dealing with large-scale codebases or systems that change frequently.