In the last step we left a little edge about the thickness of the brick material. Now we are going to scrape that off. We are not slicing, but scraping. The blade is at a ninety degree angle to the surface of the brick.
Drawing the blade along the edge on this angle will produce little bits of plastic. You can see them in the photo.  If you do this soon after gluing and the plastic is soft (recommended) then the edge will practically mold itself into shape. This happens because as the blade scrapes along, it "drops" into the mortar lines ever so slightly. This helps carry the mortar line around the edge, making a convincing corner for the chimney.