Can you post pictures? On a full tang knife you'd typically pin the guard since it doesn't usually wrap all the way around the tang, but hard to say exactly what to do without seeing the setup. If it doesn't have pin holes and needs them a carbide bit will drill through hardened steel fairly easily. Just go slow and don't use the carbide bit on the brass. It WILL hang on the soft metal and break the bit, true story. On a hidden tang I've seen solder and JB weld both used and the handle itself holds the guard on. I'll leave the more detailed advice to the experts on here.
Good luck,
Matt Toms