If you use Wa handles often, it is best to turn the blanks on a lathe with a tenon on the front. Then fit caps with matching holes to make the collar. Water buffalo is the classic material for the collars. The wood can be any wood you want that will work as a handle, but the classic woods are a white wood like Ho (magnolia) or holly, or a dark wood like walnut.
Here is how to turn a wa or D handle:
Cut a block of wood that is slightly larger than your rough handle shape … say, 1.3X1.1X6.5"" (33X28X165mm). Drill the end with a 3/8" (10mm) hole halfway down the block. Make a small divot on the other end for the lathe center.
Make a slightly tapered jam-peg chuck by turning a piece of hard wood in the lathe jaws to fit the hole.
Chuck the block on the peg and center it in the lathe. Turn a 3/4" to 1" (20-25mm) long tenon on the end with the jam-peg chuck.to be 5/8" (16mm). Make sure the shoulder is a prefect 90° .
Make a collar block by drilling a blind hole in a block of water buffalo (or contrasting any wood like blackwood/ebony). You want the hole to go 3/4 the way through the block. I like to drill a 1/8" (3mm) pilot hole all the way through the block first. Make the hole a good ( but not tight) fit for the tenon on the handle. Once the fit is right, trim the end of the tenon on the sander until it will seat right up to the shoulder with a perfect fit. Don't assemble the two yet unless you are making wa handles with round hole centers and wooding peg half shims. I prefer to cut the last bit of the collar to be an exact fit for the blade tang, which I make the height so it will just fit the hole in the handle block..
You can finish shaping the handle block to your desired size and take it to 400 grit. You can jam the collar block on the handle tenon with a little piece of tissue paper and sand it to a close match ( but leave the final bit for after assembly).
Once the blade is completely done (except final sharpening) put epoxy in the handle block hole and on the tenon ( I dye the epoxy black), and push the handle pieces together. After the excess epoxy squeezes out and is wiped off, align the collar/handle and slide the tang in the slot. A handle clamp make from a cheap Quick-clamp with a slot sawed in the front jaw is perfect for holding handles while drying. They cost about $10.
When the resin has dried overnight, finish sanding the handle to a perfect shape and finish.
TIP:
Don't put the corner facets on until the very end. It is much easier to leave it a rectangle until all the rest is done.