After measuring and marking up the brass, I drill a series of small holes in a row where the tang will fit through. Then these holes are joined and filed into a slot, using either needle files or any small thin fine file.
The tang is fitted to the guard to ensure a tight fit, and then the guard is fitted to the handle. The contour of the handle is traced to the guard, and then the guard cut out, allowing some extra brass which can be filed down later. The extra bit at the bottom was left on during the preliminary shaping stages, to help in holding the guard in the vice. Prior to actual mounting, that bit is of course cut off.
You can see I made two small errors when filing out the slot for the tang. These don’t affect the fit of the blade, and hopefully won’t be noticed once the knife is finished. I didn’t feel like starting over with the guard, so I just went with what I had.