What is the procedure for registering for NIE and where do I have to go?

You need to go to your local Policia Nacional, but you need to be sure they issue NIEs, and that it is the correct one for your area. My nearest one was in Yecla.

I took passport, two copies of my passport, 2 photos, Padron,birth certificate, Escritura - everything I could think of, though I think they only wanted my passport and photos

You can download the form online and print it off - Google NIE application form and you should find it.

They then do all the paperwork, give you an invoice, which you then have to take to the nearest bank and pay.

Take the receipt back to the police station and they will issue your NIE certificate.

Bear in mind that although your NIE is yours for life, the certificate expires after 3 months - if you always want a current certificate, you have to go through the whole procedure each time the certificate expires.

From experience, I have found that most places will accept an out of date certificate, even just your number if you keep a note of it with you (e.g. signing for a parcel)

The exception I came across was Trafico when I changed the plates on my car - they wanted an up to date cerificate.

This info is accurate based on my experience, but there is always someone to contradict you!

Hope this helps.