I had to be taught how to do this, so I thought I would share a simple technique to getting those ragged edges on watercolor paper. You can do this while the paper is wet or dry, but I usually do it while the paper is dry.
What you need:
Make your markings. You need at least two marks to make sure your line will be straight.
Begin to fold your paper by pressing down on the ruler on the line you want to tear. Then use your other hand to SLOWLY fold the paper against the ruler. Once you have a start, you can remove the ruler and SLOWLY begin making your crease.
Fully press down on the crease, making the paper flat. Make sure your bottom and top edges line up.
Open up your paper and then flip it over. Fold the crease in the opposite direction. REPEAT 2 or 3 times.
Open up the paper. Press the ruler against the crease. With your other hand, SLOWLY begin to tear the paper along the crease.