Step 4: Transform that peice of metal
I used a hand grinder but a bench grinder or hacksaw or even a metal file will work.
Second, if you had to straighten out the file you'll probablly want to get rid of the dimples that were made by the hammer. Use the grinder or file to grind down the surface until you can't tell where the dimples are. Don't worry about removing the grooves from the old file, they are deeper than you'd think and give your knife some character.
After that is done, if it was needed, you'll rough out the cutting area. Twenty to twenty-five degrees is a good angle for the knife edge. You can use the grinder to remove material initally but you should use a coarse metal file to make sure the edge is uniform in angle and flat.
Once the knife edge is roughed out, though it may be quite sharp now, sand the whole thing. Start with a coarse grit like 60 or 80 and work up to at least 150 for a smooth looking tool. You could go all the way up to 600grit if you wanted to use it as a mirror, but that's up to you.