I believe Retro Dolls US sells a lot of different tinsel, and from my understanding, you root it like any other hair, pretty much.
Uncut but crinkly tinsel can be straightened fairly significant with just the use of a blow dryer with a hot enough setting. Won't make it factory straight, but does significantly improve really crinkly ( But NOT stripped tinsel, it's done for once it's stripped ) tinsel to about 75% straightness with a lot of patience. Found that out last night, actually, when I was setting some curls on a new Taffeta and Moondust. They had really crinkly tinsel, and then I noticed as I kept the blow dryer on them, and it was blowing over the forelocks it was getting straighter and straighter!