Use the strangler fig pattern. Pick a bounded context, extract it into a service with its own database, route traffic to it, and delete the old code from the monolith. Repeat.

Big-bang rewrites freeze the product for months and risk failure. The strangler fig pattern lets you ship each step independently and roll back if needed.

A feature with clear boundaries that can be extracted independently. Auth, payments, or notifications are good because they are self-contained. Pick something stable, not the part changing every week.