tax

First - get a digital signature (makes using government websites so much easier.

Second - go to Tax Agency: Home (agenciatributaria.gob.es) select your chosen language (English?) and follow the steps. If you struggle with that you can ask for a face-to-face appointment within a certain period and they will go through your return (take all papers showing income, payslips, bank statements etc) at no charge.

You will have made returns previously so most of your information will be on file, like catastral.

Edit: DO IT NOW as the return should be in by the end of this month!

Well come to spain

We use Imont legal & tax in San Javier & have done since we bought  our house, they do a fixed yearly payment & we find them really helpful & professional. 

I still say that for your own peace of mind you should enter the your sums into the Agencia Tributaria website and check that it comes out the same as that produced by your advisers. A significant difference would need explanation! There are no doubt excellent firms doing their job well on your behalf, but there are plenty of others who are simply incompetent. Remember that they are not responsible for any mistakes just as accountants in the UK, and it will be you that has to pay any fines that may be levied, not them!