Sweet looking blade! Is the blade heat-treated yet? Aside from using the term "habaki" which has no analagous referent in English (other than "blade collar" perhaps) I tend to avoid the traditional Japanese blade terminology. For one thing I can never remember it all... and for the other it seems inappropriate unless you are following the full traditional Japanese techniques. But the habaki is totally appropriate as a feature of construction for many styles other than traditional Japanese since it provides a seat for the guard and allows the use of maximal tang width.
After heat-treating and final grinding you'll need to taper the tang in width AND slightly in thickness, and cut the notch on the top. Do is after the blade is completely finished so that you can keep the notches clean and sharply defined instead of rounded off. The peg hole looks small to me... I'd suggest 3/16" diameter if it was to carry a bamboo peg, but if you're thinking brass or stainless then 1/8" is probably okay. Reconsider bamboo though... I've used a bunch of different modern materials for the pegs, but the springiness and resilience of bamboo make it (IMO) the best material for the purpose.
The habaki itself... the simplest and most traditional is copper so you might want to start with that. Thickness... at least 0.062" (1/16") but less than 0.125" (1/8") Something around 0.090" seems ideal to me. Cut or shear a strip at least 1/2" wide but no wider than 1" that will fold around the top of the blade and meet at the bottom. Traditionally you would create a little copper wedge that fits against the blade notches and solder it in place. But it's also possible to either gas-weld or TIG weld the copper seamlessly.
The habaki is now hammered directly onto the blade to make it fit perfectly. (one reason why it must be heat-treated first) using a very light engraver's hammer. Holding the blade in hand and opening the vise jaws just wider than the tang, you force the blade down into the habaki by ramming it against the vise jaws, creating seats in the copper for the notches. If the habaki is now so tight you cannot get it off you use the hammer gently at the top and bottom edges to loosen it until it slides on and off. Trim the habaki to shape using grinder and files etc.
The technique is basically the same whether you use copper or some other material... brass... stainless... titanium. It's just that some materials are stiffer and tougher to form or trickier to weld.
