Trim Paper Carefully
Keep a broad putty knife between the wallpaper and the blade when trimming. This will protect the paper and ensure a straight cut. Also, don't lift the knife blade until you're done with the cut.
An alternative way to trim wallpaper is to use the back of a scissors blade to mark the angles on the wall covering. Then, pull the paper away from the wall and trim it along the marked line with a scissors. Only attempt this if you can cut a straight line.