It is as simple as Rob said, but I would add a little bit.
I just did one I was trying to make very accurate & happened to take some pictures.
I used a brass pin, the biggest one I could get through both blades and I peened the pin down on both sides to hold the blades firm. Then I took a piece of thin brass and tapered it down to keep the screwdriver blade from scratching the spear blade and to keep the blades parallel.
I taped the blades to a micarta jig so the blades could not rotate.
Then I used two scrap pieces of micarta to hold the blades in the vise, clamping on the blades themselves.
I used a 6 inch mill smooth file with the teeth removed from one edge and a 6 inch 3 square file, also with one safe side. That way when I have the notch cut back far enough but still need to take the tang down farther I don’t accidentally cut back in the stop for the liner lock. Also I try to keep the bastard file level with the notch on the screwdriver blade. (I make a safe sided file by removing the teeth from one side or edge with a coarse belt on my belt sander).
When I am finished the filing I grind the head off the pin on the screwdriver side so if I hit the screwdriver blade it on the belt sander (or with a file) I don’t damage the spear blade I intend to use. Then I just drive the pin out with a pin punch. Or a person could drill it out if he wanted, but the punch is faster and easier, IMHO.
Then I test fit the liner to see if I have removed enough from the tang for the liner lock to engage properly. Then I file it some more until it fits.
TIP: You can always remove more metal with the file, but you can’t put metal back on the tang if you take too much off. Take it slow and don’t over cut the notch.