Get an old file, preferably a short one. I bought a shoe box full of old files at a garage sale coupla years ago for a dollar.
You're gonna need a real hot fire, a pair of blacksmiths tongs or vice grip pliers to hold it, and a non plastic magnet. A wood fire will do it' You just have to make sure its big enough that you don't run out of fuel before you get the metal hot enough.
The name of the game is getting the metal super heated so the magnet won't stick to it. Once you have done that you have two options . You can leave it in the fire and allow it to cool slowly thereby softening it (annealing)and allowing you to belt sand off the file marks ( or beat 'em out with a hammer )or you can go straight to the next step if you aren't concerned about it looking like a file. Once its red hot and the magnet won't stick to it you'll need to hold the side that you want to use as a striker in some water til its cool. That hardens the metal and makes it easier to get a spark from.
You'll prolly wanna put a hook in one end so it'll hang off your fingers while yer wacking it with the flint.
A piece of copper pipe with the end crimped down to a small hole, makes a great bellows for the fire, by the way. Increases lung capacity as well!
Have fun fellers