If you were lucky enough to get the beckham law special status, you pay a flat 24% taxes on your Spanish income for up to 6 years. This also means that you file as a non-resident, which means you don’t need to declare world income, just what you get from Spain.
Filing is relatively easy. You fill out a Modelo 150, which the Spanish government conveniently provides in PDF form. There is a separate PDF with instructions. Google can translate it to English if you need (at least the first half).
The first part is fairly straightforward, name, address, catastral reference (ask your landlord, it’s a code that identifies a real estate property).
Part A is for your salary. Include how much you made (your company will have helpfully provided), multiply it by 24% and that’s your “Cuota integra”. Also include how much they actually withheld from your paycheck.
Part C is for any interest you might have gotten. The bank should have provided you with information about how much you made in interest and how much they withheld.
There’s other parts dealing with capital gains and real estate income which you’ll have to find someone else to explain to you.
Part F adds everything together and you’re supposed to see if you should have paid more than was withheld.
If you need to pay or get money back, fill out the “Documento de ingreso o devolucion”.
Once you are done, and if you owe money, you can take the form to your bank and they will take care of paying and filing it for you.
Otherwise, you have to go to the tax office near Letamendi and file it yourself.