Sync Mobile App and Web App without third party services

Currently building a hybrid app. using Ionic Framework with PHP for backend services

I am having a case where a user has got his profile in Web app and Mobile App as well.

If the user adds two stories as favourites (Its kind of a reading app) from his mobile app or Web app,how do I sync that?

Here are the scenarios:

1) User marks the stories favourites from web app and when I come online through mobile app, display it.

2) User marks the stories favourites from mobile app when he is offline(Now this will be stored in LocalDB). So again when he comes online sync with the server?

I know there are services like Firebase which provides syncing service.

But What If I would want to develop a Custom Syncing service on my own for my application? Is that an extremely complicated process?

If yes and it can be developed ,then how should I proceed ? A basic idea? What are the best practices that I should consider?