smithy - you are thinking of soldering like a guard, in which case you are just right.
with a habaki, you solder it off of the blade and then slide it over the tang. The solder is just to close the bottom of the habaki. It will be subject to a lot of stress, and needs ez flow silver solder (it is high temp, but the lowest of the high temps so that you won't crumble your brass). You can also get dynaflow brazing rods and braze it shut. Either work. Get white paste flux for high temp soldering and brazing.
There is a tutorial on Jesus Hernandez's website that will walk you through it. I make these often for Chinese swords, they are called tunkou. I even have to make them double-eged, which means soldering both sides together instead of one. I definitely use braze for that.