Cut the paper from the inside out in one long easy motion. Use shorter snipping cuts until the paper cuts easily again. Make sure the crease is straight and sharp. Hold the paper down with your weaker hand so that you can run the folding bone along the inside of the crease easily and smoothly. Pull the paper so that it tears along the crease.
Some papers tear more easily than others.
If you need a sharper crease, fold the paper backwards on exactly the same crease, then again the other way. This breaks down the threads in the paper so that it will tear more easily.