OK, so this isn't about Spain .... sorry Mods ... but I just wanted to say that having professional help with my tax return in France when I first filed my declaration saved me not only the fees of the accountant but a fair bit on top, and I then learned very cheaply how to fill in my declaration and legally, which has stood me in good stead ever since.
Use a professional - you never know when unpaid tax is going to bit you on the behind!