How should I set up code mapping for my monorepo?

After you have the source mapping set up properly (this is a necessary step for code mapping to work), try to find an error in your Sentry issues with a stack trace that is happening in "your code" (not in ie. node modules). There should be a button in the stack trace that says something similar to "Tell Us Where Your Source Code Is". Click on that and then put in the url of the exact file and the mapping should be set up for that path.