There are many ways, the basic theory is you expose the material to carbon for a length of time over the critical temperature and the carbon "soaks" into the material. Enough so that when quenched, the carbon % is high enough on that outer layer to harden. This works with pretty much any mild steel. Depth of the case is proportional to the soak time - it maxes out at about 0.050" with a 24 hour soak. Its not so much a replacement for tool steel, but rather offers different advantages; steel is cheaper, core is still ductile, you can block the carbon from getting to some areas so control what is hardened and what is not.
Most techniques involve some nasties, cyanide salt bathes etc best left to commercial shops. However beautiful work can be done in the home shop via pack case hardening - literally packing the work in say bone meal in a sealed container then holding it at heat for the required length of time (depends on depth wanted). This produces the fancy colours, like the gunsmiths strive for or Starrett used to put on tools.