I’d start by doing a listening tour. Understand where they are in their PMM learning journey (have they worked with PMM before? How much do they know about the role?), needs or pain points they have, and their expectations of you in the role (not all of which will be correct nor need to be fulfilled). This will help you understand how much education and alignment you’ll need to build.
Then, get to work crafting your PMM lane. Clearly specify what you will (and won’t) do, where the handoff points are, what the engagement model looks like and how people can reach out to you with requests, and who to go to for work that falls outside of your scope. Get alignment with your manager, and then do a roadshow so that all your stakeholders are aware and aligned.