I registered my car in spain via a company that specializes in this,
The first document you need is a padron which you get from the town hall to get a Padron you must be resident in Spain for more than 6 months of the year.
Once you have the Padron you can take your car with all the english docs MOT, Log book etc to and ITV centre where they will test the car (need to have lights pointing other way and fog lights on other side unless you have 2) the ITV costs about 450 as it also generates the spanish equivelent log book. Then you have to pay import tax based on value of the vehicle but this also is linked to the Padron some say iof you register within a minth of the Padro you dont pay tax some say you will i had to pay but had a padron for 2 months.
The Mayor of Alham is clamping down on Padrons and they no longer issue them to anyone just on the say so i dont know what checks they make but just to let you know that they are aware many people say they are here for 6 months when thay are not
The issue of the Padron for those who dont know is essential as this is where the Tax for the car comes from no padron no tax no tax no insurance.
Good Luck