I don't know of a tutorial, but even a description should get you on your way.
-draw the shape of the broken plaster on the foamcore.
-cut around the shape with a sharp knife, but do not cut through the whole sheet. Just cut through the top layer of paper.
-peel the paper away, exposing the foam. You don't have to peel off all the paper if it separates.
-draw the bricks in the cut away section with a dull pencil or a medium point pen.
-put a skim coat of paster on to add texture to the plaster wall. I paint a thin layer of plaster over the exposed brick to seal it as well. Be careful to keep this thin but seal any exposed foam. (Any method you use to seal foam will work for this.)
Hope this helps.