My tactic is roughly this:
Read the requirements, and write down all nouns or "players" in the document. These are usually 80% of the things you need to store or interact with.
With these things on a sheet of paper, read the requirements again and see if you can find that the things you have on paper can actually be used to do the job.
The, find their attributes and make a data model. Try to fit that into the database. Build up from there.
For web applications, this usually works for me (even for consierable size applications). As you've noticed, I didn't use terms like UML or ERD. These are just tools for communicating the model in your head with others. Powerpoint can do that, too. It's the quality of end product that counts.
