You can do it on your own in two different ways:
(i) If you are in Spain, you can fill in and download the form from the Spanish Tax Authority’s page on this link: tax office. However, the form is only in Spanish, I’m afraid. Once you have filled it in, you can print it out and pay it either in cash or by making a direct debit to your bank account. All the major national banks in Spain will allow you to do this. You should keep a copy of the form to say that you have paid for your records, but it is not necessary to send any other paperwork to the tax office. The bank will do this for you as they have a cooperation agreement in place.
(ii) If you want to make the payment via a transfer from a bank outside of Spain, then you must also send the payment record along with the completed form to the tax office at the following address: Agencia Tributaria, Departamento de Gestión Tributaria, Oficina Nacional de Gestión Tributaria, IRNR, Modelo 210, C/ Lérida 32-34 [Registro General]; 28020 Madrid. You can find more information at: tax office.
(iii) Another option is to ask the tax authority for a digital certificate, which you can then set in your computer to be able to complete the form and pay the tax. This way, you can even pay by bank card through the Spanish Tax Authority’s web page at the following link: tax office.
All the regulations regarding the 210 tax form are in the following links: