Though it's been some time (er, decades), I've forged and tempered several fire steels, and it is quite tricky. My best efforts were ok, but they weren't as good as some antiques I have, and some new ones I've bought. A really good steel is a prize, as it takes much less effort to strike a light than with a mediocre one. Stainless does not work for real firesteels.

Anything hard and sharp works with ferrocerium rods- broken glass works just fine (wrap it so you don't cut yourself).