Certificado de Sello Electrónico (FNMT) for your company

What you need to have to hand

1. A PC with a compatible browser and the Configurador FNMT installed. If the browser blocks it, try another (Edge/Chrome) and avoid corporate environments that block extensions.
2. NIF de la entidad and an email to receive notifications.
3. Mercantile documentation that proves who’s in charge (recent nota simple del Registro Mercantil or escritura/poder).
4. ID of the representative (DNI/NIE) and, if possible, the representative's certificate or Cl@ve to upload documents.
5. Time and patience: if you change device between request and download, you’ll go wrong.

Documentation

• NIF de la entidad and denominación social without typos.
• DNI/NIE del representante legal vigente.
• Proof of the position or power: recent nota simple, escritura of appointment of the administrator or a sufficient power of attorney.
• Contact email.
• For entities without personality (comunidad de propietarios, UTE, etc.): acta de nombramiento and the specific NIF.
• Proof of payment if FNMT asks for it under their current tariffs.

Step by step

1. Go to the FNMT portal: https://www.sede.fnmt.gob.es/certificados and find Certificado de Sello Electrónico.
2. Install the Configurador FNMT if the page asks. Without it the private key is not generated and you won't be able to download.
3. Generate the request: enter the entity's NIF and an email. The site will give a código de solicitud. Save it.
4. Prove representation: either upload the mercantile documents identifying yourself (certificado/Cl@ve), or visit an Oficina de Registro with your ID and the código so they validate in person.
5. Wait for validation: if it's OK you'll get an email. If something's missing they'll ask you to subsanar and that delays things.
6. Download the certificate from the same machine and user you used to make the request. The site injects the certificate into your store.
7. Export to .p12/.pfx with a password and install it on the server or application that will apply the seal. Make a secure (and encrypted) backup in a controlled location.

Tips that save time

• Same machine always: request and download with the same browser/user. If you reinstall Windows in between, you must start over.
• Fresh documents: the nota simple shouldn't be ancient. The more recent, the fewer problems.
• Test in preproduction: install the .pfx in a test environment and verify your software actually signs with the sello and the trust chain is correct.
• Expiry calendar: note the expiry date and renew with margin. If it lapses you may be stuck and many services won't warn you.
• Security: long password on the .pfx, limited access and logging. If someone copies it you might not notice until it's too late.

Common mistakes that can wreck the process

1. Changing device between request and download: “no existen claves” error guaranteed.
2. Denominación social misspelled or NIF with wrong hyphens/spaces: they will reject it.
3. Old documents or insufficient powers: you’ll be asked to subsanar and lose days.
4. Confusing the sello with a personal signature: don’t sign employee contracts with the sello.
5. Exporting without a password or leaving the .pfx on an open NAS: huge risk. If you suspect compromise, revoke and request a new one.