Now take your craft knife… – This is the point where I say ‘do as I say, not as I do’! With great care and ALWAYS cutting away from yourself, slice slivers off holding the knife at about 60 degrees off vertical around the edge where your rock faces will go. You are trying to take off the top sharp edge to make the base edge rounded. If your knife is jumping and sticking a lot then replace blade as this is the stage when your most likely to maim yourself. Also it doesn’t hurt to do this over a bin, not because the off cut will go into the bin (anything but!) but it shows willing that your at least trying to be tidy if the missus is watching.