Here is the start of my work surface. And sorry again--blogger will not let me turn my picture the right way!!! Frustration!! (Turn your head and you'll get the idea)
First put a towel under your mirror to catch any little shards and to protect the mirror and your awesome blue carpet (if you are lucky enough to have any!)
Then measure with your ruler where you want your new lines to be and mark them with a sharpie marker.
I placed my little grippies under my ruler on top of my mirror so there was no way it would move while I was scoring the mirror.
Next you want to place a drop of oil on the little scoring part of the tool.
Then you start at the top of the mirror and make sure you are scoring right on your line. THIS IS EXTREMELY IMPORTANT!! The tool has about an eighth inch edge to it--so you will need to place your ruler out 1/8th inch so you will score right on your line.
Then you press firmly on the tool as you drag it all the way down your line--applying even pressure. Make only one pass.
Then you will place the doweling under the scored part. Then with gloved hands--press firmly down on the part you want to break off. And it breaks perfectly on the scored line! Like So: