This step depends on what type of steel you are working with. For beginner knifemaker most people suggest high carbon steels because they are forgiving with the temperatures, other kind steel will require special equipment to be heat treated.

The heat treatment consist in two steps:

Hardening Tempering

To harden the steel you need to heat it up and quickly quench it, this will make it really hard (so that it will hold an edge well) but also too brittle to be used as tool. With the tempering you take away some of that hardness so that it won't shatter if you accidentaly drop it on the floor :) The quench must be done in different way depending on the steel, might be in water, oil, air or even between metal plates.

For most high carbon steel like 1095 or 1070 the heat treatment can be done like so:

heat the steel up to 800°C - 1500°F

quench in oil (any kind, I use vegetables oil)

heat again to 200°C - 400°F and leave it there for 2hr

To know when you are ready to quench you can rely on two aspect.

Curie temperature: the temperature at which materials lose their permanent magnetic properties. In other words, passed this temperature (770°C - 1418°F) a magnet won't stick anymore to steel.

Steel color: If you can do the heat treatment when the light from the sun is not too bright you can tell the temperature of the steel by his color, following color-temperature chart.

When quenching pick up the blade from the spine and quench the edge first.

If you like me are using high carbon steel I suggest you to heat up a little the oil with a piece of hot iron before quenching so that the oil is not too cold. The ideal temperature should be around 70°C - 160°F If I'm not wrong, this is done because the properties of the oil vary with temperature, and when cold it might not be fluid enough to properly cool down the blade.

The steel I made this knife out of is 1070!