Before we go on, here's a crash course in blacksmithing.
Simple blacksmithing (Bentley calls it "primitive," which is right), requires only a hammer and any flat, hard surface. It's easy to make useable wood and cold chisels, spade type drill bits for wood, and to shape screwdrivers and make simple twisted end pliers and blacksmithing tongs.
Blacksmithing is a craft where you make the tools to make more tools, a great way to build up a tool set and even sell extras to raise money for the purchase of the hard-to-build tools and materials.
Rebar works as a material for making tools. It has enough carbon so it can harden into drills, chisels, star drills, and other edged tools. You can bend it, hot or cold, to form hacksaw handles and brace-and-bit drills. And you can forge it into tongs or knife blades, and larger diameter sections can become digging bars, axes, hoes, and picks for digging.
For simple blacksmithing, heat iron or steel in a fire (charcoal, coal, natural gas or coke are best) with enough air flow to raise the temperature until the metal glows bright and red. While it's hot, you can shape the metal by hammering, bending, or twisting it. The iron or steel is hot enough to work when a magnet won't stick to it. When it cools to a dull red color, put it back into the fire to heat it again before you continue.
Avoid heating the metal to a bright yellow or white because that heat will destroy the metal's useful properties. If you want the metal as soft as possible, heat it to a bright red and let it cool slowly in the ashes of the firepit. If you want to harden steel (iron won't harden like this, only steel), heat it to bright red and then quench it in water or even used motor oil.
Properly hardened steel will be very hard to cut or even scratch with a file. The problem is the hardened steel is very brittle, it will break off if edged or struck with a tool. To make a useful tough and hard tool, the now hardened steel has to be reheated to a straw color for wood cutting tools, and to a dark blue color for metal working tools. That is called tempering.
The exact temperature and color will vary with each different type of steel, so once you learn what is best for one type of steel you can reproduce the results you want. It's a learning exercise. If it comes out too hard, reheat a little more and check again, this is safe since you won't overheat it and lose the edge holding qualities. If it comes out too soft, you can reheat it to bright red and reharden and try again.
You likely won't get it right the first time, Bentley says, so try again and again. You will get it right eventually, Bentley assures us.
[For more, here's a good guide. And please see Flamesami's comment on the Intro page about different types of metals]