Here is what a basic call to dropdmg looks like on the command line:

dropdmg --format=bzip2 --layout-name="ScreenSteps 3" --license-name="ScreenSteps 3 License" --base-name="ScreenSteps 3.0.0b12.dmg" --destination "/Folder/To/Save/DMG/To" "/Path/To/Application/ScreenSteps.app"

Notice how the layout and license that we created in Preferences are referenced by name.