To register a rental property you need to get a cedula first.  There is a technical architect in L'Ametlla who can do this. Then you take the cedula, house deeds and personal identification to the town hall to register the property.  Once the tourist registration number has come through you have to register with the Mossos. We did this at their police station in Tortosa. There is quite a bit of documentation to download, forms the holidaymakers have to complete when they arrive, notification of the tourist tax fees (to be displayed in the property) etc. Once the tourists have filled out the forms and paid their tax you have 24 hours to put the information on the mossos website.  Payment is made via the bank 4 times per year. Once the property has been registered all this info will be given to you, mostly in Catalan I'm afraid. It's quite a lengthy process but can be done  :)