The yanagi-ba WIP in the WIP-BBQ has info and photos on fitting the handle.
There are five ways to do this:
1) Cut a slot the exact size of the tang in a solid block of handle material. It will be sloppy if not exact, and lining up rivet holes can be a problem. This seems like the simplest way, but is not the best looking handle. It is nearly impossible to get the front perfect and not show the hole.
2) Make a hidden tang handle as above and drill the tang hole as close as reasonable. Pre-shape the handle to about 90%. Make a thin brass or nickel "guard" from 1/8" stock and fit it to the tang as a press fit to the point you want the handle to go to. Put on the "guard, and then glue on the handle. Trim the excess metal and shape the handle as desired. This also looks good with a matching butt plate.
3) Make a two/three piece mortised handle. Take a pair of scales and mortise out one to make it have a recess slightly deeper than the tang. Glue up this side to the tang, let cure well, drill the rivet hole (or holes) through, then sand the mortise until it is flush with the tang ( at least at the front). Glue on the other scale and drill it for the rivets. Install rivets and finish as normal. This is the simplest method, but leaves a slightly off center line on the handle. With a carefully mated handle, the line disappears in the grain. A contrasting wood butt block, or a metal butt plate is often used with this style handle.
4) Make a three piece handle. Take a set of scales and a piece of thin material that is slightly thicker than the tang.The thin material can be matching or contrasting. Cut out the tang shape in the thin material. Glue the thin material to one scale and clean up any squeeze-out. Glue the tang in this assembly. Proceed from here as in the two piece handle above. This method makes a nice centered two-line joint and looks really nice when done with a contrasting color to the scales. It works well with a tapered tang. The same method can be used to make a matching wooden sheath/saya. Micarta/G-10 does well here. Red and black is great.
5) Taking the above method one step farther, and making a five piece handle, can make a stunning Japanese style handle. The following method looks especially nice on a long handle like a yanagi-ba ( sashimi knife).
Cut and fit a 3/4" long bolster block in black wood. Make the slot as perfect a fit as you can ( just like a guard). Since the only part that needs to be a snug fit is the outer end, I rout the slot slightly oversize ,from the back, all but the last 1/8", and then carefully file the last bit until it is a tight fit to the tang. Sand and polish the front of the block.
Next, cut a thin piece (slightly thicker than the tang) of black wood in a "U" to fit the tang. Sand it until it is just about .030" to .050" thicker than the tang. Prepare two light colored wood scales for the sides. Holly works great if you can't get Ho wood. You can also do this all in black and white linen Micarta.
Glue and clamp up the white/black/white handle block around the edges. Use enough glue to get the three pieces properly assembled, but don't get any more inside than necessary. Remember to pre-drill the single rivet hole in one side, so you can drill it through after the glue dries. Insert the tang and remove it while the glue is curing. Wipe it off, and re-insert it until it comes out clean. Let the handle dry ( not on the tang). When assembled and cured the white/black/white handle block should be a perfect fit for the tang. Drill the rivet hole through the other side, and check the fit to the tang. Enlarge or oval the rivet hole in the tang if needed. Sand the end flat to butt against the bolster block, putting a temporary rivet through the rivet hole to line the handle up. When all is fitted, add a butt block of the same wood as the bolster. A few hidden pins ( or visible ones) or dowels will be sufficient for shear strength. A trick is to use 1/8" ebony dowels and drill two holes through the block and into the handle. Glue up the butt block with the dowels and clamp on the handle. When sanded clean there will be two even black circles in the black butt block. When the butt block epoxy is cured, sand it flush with the handle.
Pre-shape the handle to about 90%.
When all is ready, glue the whole thing up on the tang, inserting the desired rivet or pin. A single copper rivet is perfect for this style, or go with a mosaic rivet. Occasionally I use an ebony rivet.
Finish the handle as desired.
When any of these handles are sanded into a slightly forward tapering octagon, it is a really classic Japanese look. I really like the three and five piece handles, with the black stripe running down the center.
TIP_
I use holly wood, and have it stabilized. You can buy a whole board of good white holly online for a very low price. Cut it into blocks about 25% bigger than the size needed ( make some bigger than you think you need) and have them stabilized. While being stabilized, have about 1/3 of the blocks dyed black as well.
I cut a few of the black blocks into 1" pieces for the bolster/butt blocks, and the rest of the black into 1/4-3/8" slices for the center strip.This gives me very stable, matching grain, black and white wood. Store all this together in a closed box to keep the white wood clean.
