Once you have a digital certificate installed on your computer, you can subscribe to receive electonic notifications from the Tax Office / Hacienda.
Click THIS LINK and choose the first option in the list. Follow the instructions – in Spanish from then on.
Click the first step, and then the underlined word ´registrarse´ in the pop-up text.
You will then be asked to select a digital certificate from your computer and after that, a screen to complete with details about email address, postal address and tel. nr. will come up.
Tick ´acepto´ at the bottom and then click the button ´firmar alta´ to finalise the procedure.