A mortised tang is where the inside of the handle scales are grooved (mortised) to allow the tang to slide in to. Half of the tang thickness is removed from the inside of each scale for a snug fit. This material can be removed with a dremmel, chisel, or milling machine, etc. After a satisfactory fit up, the scales are carefully lined back up and glued together to form what is essentially one piece again. The front of the handle is checked for fit to the guard, adjusted if needed, and epoxied on the blade and guard.
I leave my handle a little large for final shaping, but already have my pins located. There are lots of variations of methods and you will find what works best for you as you make a few.
Lin