Not to be an ass, but read your textbook.
Have a good understanding of all the parts involved in DFDs "n all dat crap". Know all of your symbols, where and how they are used, their purpose, and how to draw them!
Once you know each symbol, start practicing drawing them, know that data flow arrows are linked to data stores, or how to draw context diagrams, system flow charts, etc.
You might want to write down all the symbols on a sheet, and then (without the aid of a textbook), attempt to draw each symbol beside the name.
That's all you can really do, unless you'd prefer to take a couple of case studies, analyse the scenario and create your own context, data flow, or system flow chart(s).
