Gently crease the edges of the paper to know where you want to tear. You can actually figure this out whatever way works best for you, Mako did it by folding the paper slightly, you could also measure it. The goal is to have some guide for your straight-edge without marking or creasing the paper to any great extent. Using a straight-edge (in class we used a board because we didn't have anything else but a metal ruler works well, wet the edge you want to tear with your brush. Using a "bookbinder's bone" score along the straight-edge (do not cut or tear completely through the paper). Gently but firmly pull the two halves of paper. You will see the fibers feather on the edges.