All you need is a flat file. Keep it simple.
Lay your shovel flat on the ground and step on the neck to keep it steady. Use your file to gently push away from you at the tip of the blade at a 45 degree angle. Follow along the top until you have nice, shiny, sharpened metal edge. Turn the blade over to remove any burrs.
So easy a kid can do it-with supervision, of course! :)