my suggestion is, don't try this yourself.
Take or send it to someone who knows what they are doing.
The reason I say this is because the tempering process is tricky, and subjects the metal to a lot of stress. If done improperly, you will warp the steel or even worse crack it. And I imagine this is even more so on an old tool than a new piece of steel.
This is aggravated by having a sharp edge -often you won't grind it down to its edge until everything else is done.
