This guide covers how to contribute to CPython. It is known by the nickname of "the devguide" by the Python core developers.
The official home of this guide is https://docs.python.org/devguide/, but an up-to-date mirror of this repository is also available at https://cpython-devguide.readthedocs.io/ (use the mirror if you want to verify that a recent change worked well as the copy on docs.python.org is updated only a few times a day).