I´ve never done it . But most stores will advertise the fact that you can claim back the taxes .
I don´t take back high priced items usually .. but customs at the airport would normally deal with it .
There is paperwork .. which is why I avoid it .
You should make sure you have all the relevant receipts from the store and some stores will provide you with the relevant paperwork for customs at the airport .
Maybe it would be a good idea to speak to someone at the airport ... I´m guessing you´ll be coming into T4 .., when you arrive , and get the documents so that you don´t have to go crazy filling out the details when you leave Spain .