The name of the clear plastic film is 'InvisiGRIP' by Omnigrid. Here's an easy way to cut it. Using the ruler that you want to attach the film to: place the film (which is covered with a white stiff paper) with the film down and the paper up. Now, place the 1/4" marks on your ruler both horizontally and vertically along a 90 degree edge. (you can also achieve this by cutting exactly around the ruler and then removing 1/4" from just one side and one end of the cut piece.) Keep the cut piece with the film down & the paper up, carefully remove the paper. Lastly, carefully place the ruler on the plastic, centering 1/8" on all sides.
(note: the side of the film that was next to the paper is the side that sticks to the ruler) Hope this is helpful.