I put mine in regular potting soil. I think you need to cut near a joint. I only had about 50% take. I didn't know where to cut them but some still rooted. I also rooted some in water.
Maybe someone will tell us exactly where to make the cut.

Don't know if I can tell you exactly where to make the cut, but here's one way. Our town picks up yard waste and composts it, and then residents can go to the compost site and pick it up. A friend of mine picked up some compost and mulched her beds and Costus speciosus came up all over where she used the compost. I got mine from her, as she had a LOT of it. Can't think it's awfully hard to propagate.

It can be propagated from seeds or cuttings. You can cut them into 3-6" lengths. I bury them horizontally, but you can do them at an angle. They root at the joints just like bamboo.
I like to use either rootone or/& gibberilic acid. I do prefer the combo with the fungicide.
kevin
any type of costus can be done this way

Thinking of picking up some

Costus Speciosus seed. Any tricks on germinating and growing it?

Soak them in warm water. Plant them in potting mix. Give them humidity and keep them damp. Bottom heat helps them sprout quicker. Years ago I sprouted some seeds in one of those tabletop greenhouse things with the peat pellets. I have a heat mat that I used and plant lights. Fresh seed germinates faster than older seeds. It took about 3yrs to bloom from seed.