You will need the official application form which is here:
Application formYour passport and a copy
Residencia
Padron (in date in last 3 months)
Driving Licence and counterpart
A recent photograph (if you go to the opticians by the Repsol garage in town they will do the correct size and type).
Take two photos to be on the safe side.
The payment which was €26 last year but may have changed.
NIE certificate which is not really necessary if you have residency but best to take it.
The medical which, hopefully, will still be in date when you get your appointment.
Take copies of everything.
When you go to trafico, they will issue a form stating you have applied to change your licence and they will give you your licence back
Once they have checked with DVLA UK that you have a valid licence, they will call you to make another appointment. At that appointment they will take your UK licence from you and issue you with a temporary paper one which is only valid in Spain.
A week or so later you will receive the new Spanish licence in the post.
Best of luck.