Took over product manager role from existing person now being asked for roadmap. How do I start?

Took over product manager role from existing person. Existing didn’t have a roadmap, but I’m being asked to show them my roadmap. I supported this product in the past but I don’t know how to build a roadmap for it.

I know some features that customers want from potential deals, but how do I actually build a roadmap find more features and plan out the full product. As well as plan for it budget wise etc