How to actually prepare for PHD?

I’m a recent Master’s graduate in Urban Planning and I’m seriously considering applying for a PhD. I’m trying to understand that a PhD is a major, multi-year, full-time commitment, and I don’t want to apply simply because I’m unsure what else to do next.
There is currently an open PhD call that is quite closely aligned with my interests, and it requires applicants to submit a research proposal. I have some research ideas and questions that I’m interested in exploring, but I don’t have a fully developed research abstract or a “complete” research proposal yet.
This has made me wonder: does not having a fully formed research proposal mean that I’m not ready for a PhD? Or is developing and refining the research question/proposal actually part of the process of applying and subsequently doing a PhD?
For those who are currently doing a PhD or have completed one, I’d really appreciate some advice on what I should know and do before submitting an application.
In particular:
How did you know you were actually ready for a PhD?
How developed was your research idea when you first applied?
What does a good PhD research proposal need to demonstrate?
How much should I already know about the literature and methodology before applying?
Should I be contacting potential supervisors and discussing my research ideas before submitting the application?
What are some things about doing a PhD that applicants often underestimate?

And perhaps most importantly: what would you recommend someone do in the months before applying to make sure they’re making an informed decision rather than just applying because a topic sounds interesting?