Tax

You need a certificate of fiscal residency from the Agencia Tributaria. Go to this site Tax Residency Certificates and select Request. 

After filling in a few details, you will submit the request and they will send you, by post, a letter containing a long letter and number code. About 14 or so figures in the code. The letter will arrive within a week. If you already have a clave or digital signature you can use that and just request the certificate.

You can then go back to this same link and request a certificate of fiscal residency using that code and then download the certificate which you send to your home tax office to show you are now paying tax in Spain.

This can only be done once you have paid your first income tax bill in Spain. It will mean you will end up paying tax in both countries until you submit the certificate with a Spain Form Individual but UK tax office will refund any tax you paid in UK.

Yes, you can submit your tax return online using form 100 on this site but usually best to get a gestor or fiscal advisor to do the first one for you especially as it appears you aren't fluent in Spanish.

My experience of downloading fiscal residency certificates for myself and friends is different from what marcliff says, as I simply download them on the spot, and I've just tested that for myself and immediately downloaded the certificate.

The problem with trying to get a fiscal residency certificate online is if you're newly registered in the system.

It's the norm that Agencia Tributaria don't check tax returns until four years after submission, and until they do you aren't recognised as having submitted a tax return, and can't download a certificate.

That means you have to go to Agencia Tributaria in person, ideally clutching your most recent tax return, and they'll print out the certificate for you without any problem.

Only then can you submit your form Spain-Individual to HMRC.

When you do submit Spain-Individual to HMRC, make sure you don't claim a refund of UK tax paid if you've already used that amount as an offset against Spanish tax. A friend did that and ended up with a €1000 fine.

Really? You can download these certificates at any time IF you have already registered with them. If you haven't registered or not using a clave or digital signature then you have to request one before you can get access to a certificate. Obviously, once you have registered you can download them any time as your device has been registered to accept them and other procedures. 

Waiting 4 years to download a certificate is also wrong as I downloaded mine after paying my first income tax bill and included it with the Spain Individual Form. The way you put it you could end up paying income tax in UK and Spain for 4 years which I certainly didn't.