How to reverse engineer of the system written on sql in a right way?

The goal is to move a project from legacy code written on SQL.

The first step is to realise what a project does by analysing sql procedures and sql tables.

But I do not know from what to start. What do I need to do first? What do I have to prepare or create during this reverse engendering activity?

Maybe there is an appropriate approach. Or best practice.

For example:

System has the user’s login feature.

User enters a login and a password; Systems accepts them; User is redirected on his profile page. Logic of the check that the user exists is written on SQL.

My thoughts to place an output message containing a procedure‘s name in each pl sql procedure and after the example executed to look into which procedures were triggered.

Is my approach correct or not?

Could you please advise me something)