How do I get on a project? If there is a clear answer and there is an encouraging low-barrier path to go, you have one of the most critical components for a successful open source project in place. Make it easy to use, make it easy to contribute, make it easy for users to become contributors.
The following stepping stones form the path of onboarding. They are not in a specific order. Each project will pick and arrange them in a way that is suitable to them.
For people new to your project it can be hard to find their ways. Mentoring is a great practice to make this easier. Put experienced people at their side to help them getting started. Some of them will grow into contributors and mentors themselves.
Quick responses keep new contributors on board. Clear expectations give people confidence.
Give people new to your projects some dedicated entry points, where they can start. This can be issues labeled as "good first issues", junior jobs, quick start documentation, or anything else which helps with orientation.
How users become contributors, how contributors become maintainers. Projects which establish a healthy funnel there, so that new people stand up when others leave and there is momentum to grow the community, have the most necessary source for success: Motivated people.
One important part is clarity about maintainership. Who are the maintainers? How do you become a maintainer. This includes the responsibility to gracefully step down as maintainer and pass on maintainership to somebody else.