As I mention at the end of https://github.com/aardappel/goose/blob/master/README.md I essentially had it use a previous compiler of mine (Lobster) as a template.

Even if you don't have such an example, pick a project that you think has "good code" (even if it is not your own), point the LLM at it, tell it to have a deep read of the code, and come up with a very in-depth coding guideline in a .md file with code patterns etc. Then put that .md in your new project and tell it to follow it.