This isn't 100% spot on, but fairly close:
http://www.dakkadakka.com/dakkaforum/posts/list/243963.page
scroll down until you see the section where the tutorial shows how to do placeable terrain (trees and such, made from wire)
As for scorch marks on a tank or some-such:
"Scorched brown with calthan brown and graveyard earth highlights looks pretty good. Throw a little boltgun metal in there for a realistic approach."
~Samus_aran115