Heat treating steels is a huge subject. What you do depends on what alloy you have and how you want it to perform.
In general, carbon steels can be made hard by heating to a dull to medium red, and then cooling quickly, or "quenching". The quicker they are cooled, the harder, and more brittle, they will be.
After that, the steel can be heated to a lower temperature and cooled slowly to relieve the brittleness, but maintain most of the hardness. That is tempering.
The specific temperatures, procedures and quenching medium (usually oil or water, depending on the alloy) for hardening and tempering depend on the alloy and the specific properties desired.
A problem with big pieces, like a vehicle frame, is that you can not readily harden and temper one area - at the edge of the area you heat and quench will be a band that was not hot enough to harden, and is "annealed," or made soft as possible.
I think that Jeep frames, at least older Jeeps, are all in an annealed condition, which is what allows one to weld to them without any special after treatment.