In order to file on-line, you need a user certificate which must be installed in your browser in order to generate the electronic signature. Not sure how you obtain one, though, sorry. Best bet is to ask at your relevant Hacienda (Agencia Tributaria) office how to go about it.
Personally, I find it less hassle to pay someone, like the previous poster does. I also pay €35 for this (I am resident, but my return is pretty simple too)
I must have missed this thread first time round, but have read through it now. On the question of other assets in Spain, and how to include them on form 214 - you don't. Form 214 is purely for non-residents whose "estate subject to tax in Spain is exclusively comprised of an abode". (source: Agencia Tributaria) If you have other assets, you should probably be using Form 210 or 215. As for interest on bank deposits, if this has been paid gross (i.e. with no tax deducted at source) then it is probably because as a non-resident the interest is exempt from tax. But in this case, you should obviously be declaring it in your country of tax residence and paying tax on it accordingly. Sounds like a handy little loophole, but exchange of information has increased substantially in recent times, and it's probably not worth taking a risk.