The following is a tutorial from Tai Goo:
Here’s a very simplified version of making a knife blade from a file… not the only way or the best, but should give satisfactory results for newbies and help establish a starting point.
#1. Get a suitable Nicholson file.
#2. Soften the file for stock reduction by taking it up to a black heat (day light in the shade) holding it there for a minute or two and then let it air cool. If you start to see even the faintest glow, take it out of the fire until it goes away and then go right back in. Repeat this process several times at any point prior to hardening, if necessary.
#3. Grind or file off the teeth (especially along the edge).
#4. Grind or file the blade to shape.
#5. Sand the blade to 100 grit parallel to the edge.
#6. To harden the blade, take it up to non magnetic, plus 50-100 degrees,… usually “cherry red” (daylight in the shade). Hold it there for a few minutes and quench it in fresh warm vegetable oil.
#7. Go immediately to the temper while the blade is still warm to the touch. Temper at 400 - 450 for an hour, (use a good oven thermometer) allow to cool to room temp., and repeat 2-3 times.
#8. Clean the blade off with sandpaper or a water stone.
#9. Sharpen the blade.