As above, I presume that this is/will be a UK vehicle.
First job - change the headlights and rear fog/reversing lights. LHD Headlights are available on t'internet for a considerable sum, or go to your local autodesuaces (breakers yard) - 40/50 euros per pair. You may be lucky and have a car with a pair of fog/rev lights (eg. most Citroens, Renaults etc) in which case you will be ok, otherwise breaker's yard again.
Next - perito's (engineers) report, or Ficha Technica Reducidia to give it it's correct name. This can vary from 80 to 175 euros - I found an engineer on t'internet who did it on-line in 24hrs for 80euros, including an electronic copy to the Itv station to confirm authenticity.
Book an Itv (M-o-T) at your local station - make sure you tell them you need a test for matriculacion as this is more stringent and more expensive than a normal test.
Go to the Town Hall - or in this area, the Suma office, and pay the Spanish equivalent of the road tax - varies with emission levels and engine size.
Then on to the local office of the the Agencia Tributaria and pay the Impuesto de Matriculacion.
Finally, you go to Traffico!!! Take all of the above paperwork, old V5 in the UK number, insurance certificate (Spanish), passport, padron, inside leg measurement and anything else you think you may need. This used to be a long and complicated rigmarole, taking two days. Now (here, at least) you make an appointment on t'internet, turn up at the given time and all is done within half an hour. Them, clutching a fistful of MORE paperwork, go to a local accessory shop and get your new plates - dead easy and great fun if you can laugh at the Spanish love of paper!
I have just done a Ford Focus 1.8 diesel 2007 and all of the above cost me just under 400euros. Bear in mind if you employ a professional agent to do it for you they will charge you from 500 to 800e, PLUS all the above fees.
Good luck and don't be afraid.
Steve