Welding will mess with the mechanical properties of the blade steel, at the guard joint, although some post-welding cycles could be performed to mitigate the effects somewhat. A better way as JT states would be to forge it all integral if you wanted a one-piece knife like that. Brazing is not very common, as heat treating would have to be done afterward and the heat involved for heat treating and brazing are fairly close to one another.
Having to slot, pin and solder a guard is a very common method, although anymore it's just another reason I prefer hidden tang designs. I did make a fair few knives that way though. You'd peen the guard on with a couple of pins, tight so already it won't move, and then seal up the joint with solder. I recommend stay brite solder and sta-clean flux, the most common combo in use probably with knife guys. Do some research around this site, and around other forums and YouTube if you want to learn how to solder. Or, buy "How to make Knives" the old standby written by Loveless and crew, which has a good description of the process. The blade will already be heat treated at this point, and the solder used will flow at or below the temper temp of the blade steel.
