How do I glue balsa wood?

That would very much depend on what it is you are trying to glue the balsa wood to, if you are gluing it to another piece of balsa wood, I would say either balsa cement or PVA glue.

This would be the same for gluing it to cardboard, paper and other woods while PVA glue only for the polystyrene foam.

If you were trying to glue it to plastic, you would need to use cyanoacrylate glue (super glue) or an epoxy glue.

If you were trying to glue it to fabric you would need to use a rubber-based contact glue.

If you were trying to glue it to metal, glass or cork you would need to use an epoxy glue.