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.