[znt]

If databases are in play, instead of trying to understand the business logic, have a go at understanding the database structure first.

What are your tables, how they are related etc.

Then look at the modules that interact with the data layer directly and move up.

This is a language agnostic approach though, but it has worked out well for me.

[baremetal]

That method works well for initially building the system too. Start in the db and model the relationships and then move up.