Get the right size! biggest tip right there. If you have a decent hardware store in town take the nut there and maybe they can help ya determine the size. then confirm it is the same as the spindle. It may be a wrong thread that caused the problem. Then buy the dye.
A (retreading) die will be tapered (inside). Start it with the big end on the spindle. DO NOT FORCE IT. make sure it is straight and it will self start.
(well you still have to turn it, it won't turn itself) Sorry I can't help being a smart a$$. Nothing derogatory inteneded.
Once you're convinced everything is correct get a wrench and start turning. While yer turning have someone get online and order another spindle.
(Dammm, there I go again)
Use a little oil. Cutting oil is great, but I'm guessing this is a one time thing, so use any oil. (motor oil or the like, not cooking oil) Turn the dye reverse every revolution or two to clear the chips from the process. Candles, incense, chanting and prayer are always helpful.