it depends on the sketch and the tools you KNOW how to use

for example :
this idea has been floating around my head and hard drives for a bit
Dune's Arrakis
starting with something like this
http://feyd.rautha.free.fr/Dune/Arrakis%20map%201.jpg
and converting it to a height map for 3d rendering


or

something like from this post here
http://www.cartographersguild.com/sh...384#post261384
using "wilbur" to make a height map
and from that a labeled map


then there is the scan the images into the computer and work on them with The Gimp or in photoshop