You need the application form, available from the web site http://www.mtas.es, a padron, and your old card - they now only issue the new certificate.

You will also need to pay 6.70 using a form 790 at a bank before 11.30 in some cases (not available online because it is carbonated multi part) available from the office who issues your certificate (ie Comiseria de Policia - you can search Google maps) The price is the same as the Spanish are charged for their DNI cards - so no whinging!!

Kiss your cherished card good bye!