Before going to AESA, you have to make a claim with Iberia. This should be the link:
https://www.iberia.com/web/program.do?menuId=CEATCL
Then, when they deny it, you can go to AESA and attach Iberia's response (or a copy of your complaint if they do not reply in a month).
It is a simple process (you only have to fill two forms, Iberia's and AESA's), but you should be prepared for it to take months, even if the delay was Iberia's fault and you are entitled to compensation.