What should I do to legally stay ?

As an EU  national, if you move to,  or intend to stay,  or do stay 90 day permanently, then you are required to register on the  EU Citizens Register at the National Police Station that covers the area where you are living at the time of application.   Even if that is just a hotel or hostal for a night or two.

All that is needed is:-
1.    the application formr, an EX 18 and a copy,
(download at   
http://extranjeros.empleo.gob.es/es/Mod … itaria.pdf

2.    confirmation that the payment has been made to a bank, 10.60 euros, 
              complete the 790 payment form on line and then download and pay at any bank, Download from:-
              https://sede.policia.gob.es/Tasa790_012/ImpresoRellenar

3.     passport and a copy,
4.     If a worker either proof of your employment in Spain or a job offer,
5.     if not a worker then proof of full medical cover and sufficient income or savings

MINIMUM INCOME Requirements
1 person:    savings of 5,136.60€ or monthly income of 366.90€
A couple:   savings 8,732.22€ or monthly income of 623.73€at least 366 euros p.m. income. 
 
MEDICAL COVER
Either private insure or if a UK pension medical cover in Spain provided on application to DWP for form S1

That is the instruction from the Ministry of the Interior and is the same throughout Spain.

Info for 790 (if you look at the form this info should be obvious)

IDENTIFICACIÓN
N.I.F./N.I.E. (*)                         leave blank as you are applying for an NIE
Apellidos y nombre o razón social (*)      surname, followed by forenames 

Domicilio                                  details for your address (i.e. where you living / staying on the
                                                   day you make the application)

Tipo de vía (*)                   Type of ‘road' you live in  Calle, Plaza etc
Nombre de la vía pública(*)   Street name
Núm. (*)                                     Street number of your address
Escalera                                      staircase number if appropriate
Piso                                             floor level if appropriate
Teléfono                                    telephone number, fixed or mobile
Municipio (*)                            Town in which you live
Provincia (*)                              Province
Código Postal (*)                      Post code

(*)                                               Item must be completed     

Asignación de Número de Identidad de Extranjero (NIE) a instancia del interesado.



DECLARANTE            Person making application
Localidad (*)            The town in which you signed the form (that is a usual requirement onç
                                   official papers)
Fecha (*)                  The date of signing

INGRESO    amount of PAYMENT
Importe euros
€
Forma de pago (*)      how you will pay at the bank
En efectivo                 Cash   
E.C. Adeudo en cuenta  from your bank account