I would suggest starting here with this article for the actual sand basing, where you can choose between a stander or more coarse base:
http://uk.games-workshop.com/warhamm...rain-basics/5/
Then, head to this link, where it describes how to paint the sand to make it look like a proper ashen waste:
http://uk.games-workshop.com/warhamm...ain-basics/12/
Hope this is helpful!