Good question! It can be quite a feat - one thing to look into is if there's any frameworks available that fit into the other tools you're already using. For example, if you use the Apollo framework for GraphQL, a lot of work has been done to make it possible to get your application to work offline. Or you could look at a library like Automerge and see if that can help you deal with those pesky conflicts.
Either way, you should expect to invest some significant time. What we did at Kitemaker was to incrementally improve pieces to get ready to move to offline-first before we took the plunge and tackled the really big pieces.