Not having nut files, Trevor's method is probably the easiest (and cheapest!).

Just be sure to keep the nut flat and square when you're sanding so you don't get an angle on the bottom. Put the sandpaper on a good stable, flat surface and move the nut along the paper. Don't try and sand it in your hands IYKWIM. You may want to tape the sandpaper down to hold it steady, but that's up to you.

It's not a bad idea to mark the nut so you can track your progress.