Residency, I believe, is different from registering on the electoral roll. One states you are resident in the country with all the benefits that has.
The other is the same as the UK gives you the right to vote.
It is many years since we first registered but it was quite simple. Just go along to your town hall, ours was very helpful and glad we registered. Apparantly they get funds from central government for each person registered.
As you know everywhere is different but we took the usual bundle of papers -Residency/NIE, Passport, Copy escritura etc.
It was very quick and gave us the right to vote.
Hope that helps