Dotty,
When you insert a black DVD disk into your external drive, Mac OS X should "mount" the DVD to your desktop. By mount, I mean a picture (icon) of a DVD or CD will appear on your desktop. If you double-click on this icon, a window will appear. You then simply copy and paste any files you want to burn into this window. There should be a "burn" button also listed in that window.
It's pretty straight forward hopefully...
Good luck.
Edit: it just occured to me that you may already know that (hence your question...) sorry if I dumbed it down.