You should disassemble the post hole auger before you begin because the blades on a pair of post hole augers don't open very wide. All you need to do to disassemble a post hole auger is to loosen the wing-nut on the side then remove it and push out the bolt that holds the blades together.
Clamp the First Piece
Take one of the post hole diggers and clamp it down into a vice. You should clamp it down to look like an upside down u.
Clean the Blade
Use a high pressure spray nozzle and water hose to spray away any dirt or grime that is on the blade. Now you should use a wire brush and scrape the edges of the blade free from any debris or obstructions.
Grind the Damaged Areas
If there any damaged areas on the blade you should use a medium grit wheel angle grinder to fix this.
File a Sharp Edge
Once you have the blade prepared use a sharpening file to begin filing the blade. Hold the file at a 45 degree angle and slide the file back and forth in a front to rear motion. Continue doing this across the entire surface of the blade. From time to time check the blade to make sure it has been completed properly.
After this all you have to do is reassemble the blade.