How do you use a DFD Model?

Think of it like this, draw a box on a piece of paper and right "start" in it. Then what happens from there? What if the user clicks a button? Well lets say the clicks the maximize window button. Well what happens? Draw a line to another box with "Maximise window function" in it. Draw a line from start to the new box and write a little note beside that line "Clicked maximise window button." And you continue doing this for any function that you want to include. This is great for start a new program or debugging a program you have already wrote.