On mortise and tenon construction.
The easiest method would be to use 1/2" or 5/8" dowel stock readily available (maple, oak, poplar, it really doesn't matter, whatever is easy for you to work with). The stock will need a slot cut in for the tang. It can be done with a handsaw, or a bandsaw, but the bandsaw method will be a little tricky, so one will have to think of a method to keep the round stock stationary. You will need a jig for that.
From that point on, one would need to pre-drill 1/2 diameter hole in the handle (calculate the depth by subtracting ferrule+spacer from a length of the tang from machi. Then pre-drill horn about 2/3 the length. If you have a XY vise, you can then change to a smaller bit and drill side by side holes through the remaining 1/3 of the ferrule. Turn ferrule to the opposite side, and clear the tang slot with either milling bits (ferrule is clamped in the vice, don't do it while holding it in your hand), files, or a broach.
Your metal spacer will have same 1/2" diameter hole as your ferrule and handle.
Assembly. First, glue in the slotted dowel, making sure the slot is square. Once your epoxy cures, you can assemble the handle with a spacer and ferrule. I would use a flat piece of metal stock, 1/2x1/8 to align the dowel slot with the tang slot. Let epoxy cure for 30-40 min (get slow curing epoxy) then clamp the handle. A choice of clamp is important, so as you tighten it, it wont shift the alignment.
Once the epoxy is cured, you can proceed shaping it. Strength of your handle will be derived from the dowel, so even if your spacer heats up, handle will hold in one piece.
Horn slot can be shaped best with 1/8 broach, files or milling bits. I would not recommend to clear it with the drill bit or try to clear the slot while holding a piece in hand.
Disk sander with a miter slot and miter gauge is essential for squaring.
Hope this helps.
M