Hi there,
In order to pay the Non Resident Income Tax (IRNR) you need to complete Form 210 online within the Agencia Tributaria website. It is available in English and when properly completed online you can press the button "Validate" which generates a 5 page .pdf document which you can then save to your computer and print off. At the end of this form there is an option to pay in cash which means you bring all forms to your Spanish Bank where they will charge the amount to your account, keep one form and return the rest to you with the Bank stamp thereon as the official receipt. The second option allows the Agencia to do a direct debit to your account - you must put in the account IBAN for this. You must select a payment option before validating the form online.
I understand you may also pay online if you have an electronic "digital signature" acceptable to the Agencia...however that is another day's work!
The Form 210 is only for use in certain circumstances, for instance an owner with one property which is owner occupied and not rented. The full regulations are available on the tax website.
Hope this helps