All you need to do is to, first, decide where to cut the shrink plastic. I had to cut mine in half lengthwise, so I placed my shrink plastic on my scoring board and measured the width. I divided the value by 2, and that gave me the position where I needed to cut my plastic.
Place your long ruler on the midpoint (I didn't place the plastic on the 0" mark, so I had to shift my mid point accordingly), following the groove at that measurement. (You simply just have to remember where you need to place your shrink plastic and the point where you need to place the ruler for the other sheets of shrink plastic that need cutting)
Then, take your craft knife or the sharp tip of your scissors, lightly score along the midpoint, using the ruler as your support. (The key here is to use a light pressure. The plastic should score very easily and accurately with the help of the ruler.)
You should get this nice scored line on the shrink plastic.
What you need to do next is to fold along this scored line.
It should fold nicely without much fiddling.
Then, unfold, and refold in the reverse direction.
The shrink plastic should snap into 2 easily and cleanly!
There you go! It's really simple and fast, and there's no need for sore hands! (Trust me, you'll get sore hands trying to use scissors to cut tons of shrink plastic. Ditto when trying to keep a good pressure when using a craft knife to cut plastic!)