LINK to forms 036 and 037, used to register a business with Hacienda / Agencia Tributaria. The general information is available on the Tax Office website in English, but the actual forms are in Spanish.
Possible online with Cl@ve or digital certificate installed on computer, or print out as pdf and present at nearest tax office in person.
Link to online registration on RETA, the social security ‘regimen’ for self employed Autonomo workers.
Scroll down to ‘Register for RETA’.