To be quite honest if you don't know what you are doing its a bit of a nightmare and is totally worth getting a gestor to do it for you. I speak from experience!
You will need to surrender your logbook and change your lights as the stickers just won't cut it during an itv test. The gestor will go to Malaga trafico for you and come and inspect your car to tell you what you need to do to get it changed over. I needed to buy new lights and because I have a car with only one reverse light and one fog light I had to get the wiring changed around at the back. (Luckily my fiancé is fairly good with cars and he did that himself after some Internet research)
The gestor will tell you how much import duty and road tax to pay and you pay it to them and they sort it all out. (If you have a low emission car the import duty is zero, which it was in my case)
You then have to go to the itv and get the car tested. Once you've done that and passed you will be road legal. And you'll have your Spanish plates.
I used Katie at Castles in Sabinillas and she was very good.
Hope this helps.
Daniela