Use the decorative scissors to cut strips in the piece of paper - leave about an inch uncut at the end, so that you'll have something to hold on to. You want the piece of paper to still be held together when you run it through the crimper - much easier than running each strip through individually!
Then crimp the whole sheet, and cut off the inch at the end so that the strips separate.
Different types of crimper will produce a different look. Try crumpling the paper into a ball in your hand after crimping it if you want it to look even more crinkled. You can also cut the strips shorter or try longer sheets of paper, depending on the look you're going for.