I am not a geologist nor have I studied agricultural development, but I have always believed that sand deserts came AFTER a topography was there...I would suggest coming up with an interesting topography for you area and then cover the parts that you "don't need" with sand....just come up with a decent back-story if you plan on revealing signs of lost civilizations later...

Of course, if you need a geologically realistic justification for your map....please disregard the above.....it is in NO WAY intended to be a statement of actual geological processes.