First I laid out a grid of same size pages on a pieces of paper, but instead of using a pen or pencil I used a tool to score the lines. I don't want ink or graphite on the edges of the pages, and scoring helps fold the pages later.
Then, using a new (clean) paintbrush, I brushed water onto the lines I wanted to tear.
Let the water soak through, then slowly pull the two sides away from each other.
That, my friends, is how you tear paper where you want it to be torn; weaken it with water. You don't need to score first for the water to work, I was just marking my lines. This technique works fantastically freehand, or to tear on a curvy line. Thicker paper, like cardstock, requires multiple passes with your brush, keep checking the back side until the line has soaked through. Thin water lines are precise, thick water lines yield jagged tears. If you want to stain the edges a bit, use tea or coffee instead of water. Play with it, and have fun.