When I grew tired of resharpening blades after every other mowing, I started taking every blade I buy and cutting out 1" strips and inserting L6 edges properly heat treated for high impact cutting.
That's as cool as the other side of the pillow.
Now, I'm going to play devil's advocate... IF you are on the other side of the planet and the cost of having new cutlery-grade steel shipped to you is prohibitive, and IF you don't have a reliable way to fully HT it anyway and shipping it to a pro is too pricy, and IF you have some sort of belt grinder, and IF you do have a toaster oven or regular kitchen oven that's more-or-less reliable, you can make a pretty good knife out of a good file (American-made Nicholson or Simmonds; I can't vouch for others. DO NOT bother with cheap case-hardened India or China files). The point is to use a minimum of equipment and readily-available material.
The process is as follows:
1) Grind at least part of the file smooth so you see clean, bare metal. Re-temper the (very hard and brittle) file at 350-400 degrees F for one hour, let cool to room temp, repeat. This will bring its hardness back to a more manageable level - hard enough to keep a decent edge but not so brittle. On the bare spot you ground, you should now see a pleasant straw-yellow color, perhaps even bronze.
2) Further temper the tang area with a plumber's propane torch, taking care not to over-heat the blade itself. Keeping the blade area submerged in water while you heat the tang helps. You can turn the tang blue, purple or even heat it right up to orange, as long as you don't heat the blade that much. The purpose of this step is to soften the tang enough that you can drill holes for pins or bolts, while leaving the blade nice and hard.
3) Grind to shape and bevel on your belt grinder. Keep it cool! If the blade edge or tip turns blue you've heated/softened it too much and it won't hold an edge. You could do this with a new file but good gravy it will take a while... remember, this is still hardened steel.
4) Assemble and finish as usual. ,
Is this the best way to make a knife? Of course not. But it's cheap, easy and almost always preferable to the junk on most store shelves. If you're interested in making a decent file knife, let me know and I'll provide you with more links and info.
Having said all that, new steel and proper HT is definitely the way to go if you can. I still make the occasional file knife (hey, files wear out and I hate to throw anything away) but I don't warranty them to the same extent I do my "real" knives.