this guy can teach you alot if you get trough the ramblings:
http://www.youtube.com/watch?v=qg3jDp7s83s
You buy blue stuff styrofoam or pink works thats used to isolate buildings,cut out pieces,cut lines of wall tiles or whatever you want ,use a blunt pen to widen the cuts,push down certain corners etc to make it irregular.
when your happy its time to make it hard.
basicly you mix transparent/white glue,water,some acrylic black paint to get grey color,plaster of paris,acrylix/latex filler.
all the ingridients must dissolve in water or it wont work.
you mix to to a goop and then you paint on in layers let it dry paint again.
mixing is trial and error.
you keep the goop in a container closed so it dont dry up on you.