Re: Information is a dangerous good
The catalans need to organise a change to the spanish constitution to allow communities (or provinces or whatever) to secede from the union.
Once they have done that, via the normal political channels, they can have a local referendum to decide whether they want to leave or not. the same would apply to all of the other communities as well, of course.
At the moment, their "referendum" has been declared illegal under spanish law, and should therefore not take place.