I've used a couple different ways. One was to use a thin metal sapphire nail file that I found at a dollar store. Works ok, and is thin enough that I can get reasonable far into the crotch of the scissors (what is the proper terminology for scissors, anyhow?).
Another method I've used more recently was a small tungsten carbide scraper blade. It's about an inch and a half long, about a quarter inch wide, and about 1/16" thick, sort of trapezoid in cross section. It easily sharpens the blades the complete length with some care. I have a Corona tungsten carbide sharpener that also does the job in the same way.
The most involved way is to disassemble the whole thing, which allows the blades to open up. But this is more work than the thing is worth.
