Open the paper roll. If it is taped shut, unhinge the tape and unroll the paper. Be careful when opening a paper roll that is sealed with an adhesive. You don't want to create too big of an unruly rip as you might not be able to use that portion of the paper.
Measure out with a ruler or tape measure exactly how much paper to trim.
Mark your desired length of paper with a pencil. Draw a straight line using a ruler or the edge of a book.
Cut the amount of paper you need using scissors. Use a box cutter to speed up the process if you have to cut a lot of paper.