When fitting a guard to the tang there are several issues. Lets try and separate them:
1) The tang must be straight, finished to at least 400 grit, and hardened. The tang should have sufficient taper toward the shoulders both in width and thickness. I like to make the tang end about half the ricasso width and thickness. Don't make the mistake of fitting the guard and then doing HT, as once cleaned up post-HT, the tang will be smaller. The blade should be completely finished except sharpening when the guard is fitted. Any sanding done after the guard is fitted will make the guard fit sloppy.
2) The guard slot should be centered in the guard. It is very difficult to cut and file the slot and end up aligned left to right and top to bottom with the guard shape. The way to solve that is to make the slot first and then shape the guard.
3) The slot can be made many ways, but the simplest is to drill two holes that are about 80% of the desired slot width and connect then with cuts. Mark the centers of these holes at the width of the shoulders apart and drill the two holes that will be the ends of the slot. If the tang measures .100 at the ricasso/shoulders, then use a .080" bit. Use a metal straight edge and draw lines along the outside of the two holes and mark the slot sides.
4) Drills don't make square holes, and filing squares corners in slots is always a problem, so why do you have square edges on your tang???? When making the tang pre-HT, file the tang upper and lower edge in a radius. This will make fitting the tang to the slot much easier.
5) Cutting the slot - There are many ways to cut the slot, You can mill it in, drill a lot of holes and cut out the webs, or just saw two straight lines between the end holes. While it takes a bit of practice, using a jewelers saw, #3 blades (buy a gross of quality blades), and some eye-hand coordination will cut the lines out cleanly. Start at one hole, saw slowly down the inside of the marked line, and when you reach the other hole, reverse the guard stock and cut back to the first hole. When sawing with a jewelers saw, you always saw away from you. Once the slot is cut out, use thin files to slowly make the slot fit the tang. GO SLOW in filing! You can take more off, but can't put even a little back on.
6) Using a jewelers saw is not like using a coping saw. The teeth point toward the handle, and the cut is on a pull stroke. Attach the saw blade to the handle end and tighten the lock tight. Slip the guard on it and attach the other end of the blade. Put the blade in with plenty of tension. I place the handle against my chest with the other end against the saw arm, and push with my body while I tighten the lock. It should sing like a guitar string when plucked.
The metal should be held down with your fingers or a small clamp against a sawing arm. This is just a piece of wood with a long "V" slot in the end. Screw the saw arm to the workbench and sit in front of it when sawing. Place the saw so the blade is in the arm's "V". Saw slowly and only on the down stroke. Slightly lift the saw away from the cut on the return stroke. Sawing back and forth will kill the teeth and the saw will start to cut in a curve. If sawing slows down, or you can't make it cut straight, change the blade. It is false economy to try and use a blade once it is shot. In the beginning you may use a dozen blades to cut one guard. Later on you may use the same blade for several guards. A little "blade lube" with a candle will also make the blade cut smoother and last longer. Obviously steel cuts slower than brass, and you can only cut annealed steel.
7) Once the slot is sawn out and the filing is getting along, start fitting the slot to the tang. It would be a rarity that the tang was perfectly square in all directions, so the tang slot has to be fitted to the peculiarities of the tang. Blacken the inside of the slot with a sharpie and work the guard on tight, remove it and see where the sharpie rubbed off. That is where you give a few more file strokes. This will allow you to slowly make the slot seat down toward the shoulders.
8) Quit when you are almost there. When filing in the guard slot, get it so it goes to about 1/4" from the shoulders. At this point, you can start shaping the guard to the shape you desire. The slot will be dead centered because you made it first.
9) Once the guard is shaped and sanded, and you are ready to install it, clamp the blade in a vise between leather pads, with just the ricasso showing. Place the guard on and slide a block of soft wood over the tang ( drilled or slotted to fit the tang). Tap the guard home gently. If it won't go without excessive force, remove it and file a few strokes on the offending spots. When properly fitted it should tap down just snug. Solder it on and you are ready to finish the handle and knife.
They're like strands of hair. I guess I understand when told to buy dozens of blades at a time. I take it they break often? What about scroll saw blades? Would they work also?