In the past I tore paper very carefully by hand to keep it in a 'sort of straight line' but then I found a paper edger which is actually a ruler that you can use as a guide (details below). This makes life much easier but I have realized that if you want a neat edge then it is best to place the ruler on the back of the image topper and then tear. If you look carefully at this card you will see the difference in neatness between the white and blue edges. I forgot this little rule when tearing the white but corrected it before tearing the blue and it is noticeable, well to me anyway!!!
Jef Paper Tear Edger LP-06
Scrapes of paper: blue and pink
Card stock 180gsm in White and black