They way I do it is to make a straight, one inch cut with scissors. The grip each side of the cut as firmly as possible and give a quick, jerk-like rip. Then when I have enough room to grip it with the whole hand I do so and go for longer rips. Each time moving my hands up to the point of the split. You've got to put a little umph into the rips. It will come out straight. I'm sure it has something to do with the twill weave. Then there's the aroma of ripped wool tartan.