Making a good sail from "first principles" is almost impossible. Probably the best way to start is by getting an old sail (that's not too blown out) and making patterns for each panel from it. The best way to do that is by ripping apart the seams, but you can probably do "OK" by tracing the panels. Remember that the accuracy in the panels is probably on the order of 1mm, so you want to be very careful when you're making your patterns and cutting/stitching the duplicate.
Another excellent way to get started is with a kit from Sailrite. I notice they list the DN on their website:
http://quotesys2.sailrite.com/ShowAd.aspx?id=4323Roger's advice about using double-sided tape to hold the panels together while you're stitching them is very good (I don't know of any sailmakers that don't use it). It also allows you to more carefully align the panels together (remember that 1mm tolerance) than you could if you were just trying to feed the two pieces of cloth into the machine.
In general, Sailrite is a good resource for sail-making supplies.
Just out of curiosity - what kind of sewing machine are you using?
Cheers,
Geoff S.