The two easiest methods, that still do a very good job, involve using sand paper or aluminum foil. In both methods you literally just cut the material using full scissor strokes each time. If you want to sharpen them more “professionally” and you have a grinder about, there is a video below showing how to do that as well.
With the sandpaper use a relatively fine 150 or 200 grit sandpaper, cut with the rough side down as illustrated in this video:
With the aluminum foil, take a nice sized sheet of aluminum foil and fold it a few times to get it nice and thick. Now cut thin strips of the aluminum foil with the scissors. Easy!
Expand for References: