My wife and I have just sorted our income tax out, in the end it was not a big deal. We found our local tax office, went in and in five minutes we had our forms and labels. What we did was have the right questions typed in Spanish on a piece of paper which we just hand across and not problem, we got what we needed.
All the posts on EOS with regards to filling out the forms were of no help and we did not trust all the pandering people, offering to fill it out on our behalf.
We got a gestoria to fill out the form for us and they only charged for that service and not try to charge to "represent" us for a full year, which did not need because we sorted everything else ourselves.
Our advice is get a professional to fill it out the first time ( please note that we have been informed that mistakes on the forms, especially with regards to the monetary amount will incur a penalty but if the professional makes the mistake then they pay), then you can easily fill out the form from then on.