Working with paper sometimes consists in tearing it. But mostly you want it to be torn in a particular way and not crooked ;-)
Creative Paper Crafts gives lots of tips, tricks and ideas, one of them being a how-to on tearing paper.
I tried this technique several times and it always worked out exactly the way I wanted.
Thats what I call a good instruction ;-))
Links:
Creative Paper Crafts: a website where I can surf for hours! ;-))
Tearing Paper