Its basically similar to the UK.
I married my Spanish partner in Spain, but we lived in the UK, I believe the process is the same.
You contact the Civil Registry in Spain who will prepare the application.
Contact the UK registry as you need proof your single. After the wedding you`ll need to register the wedding in the UK as well, I think, I did but we were living in the UK.
You`ll need birth cert, passport, cert of residency (unless you live abroad), cert of consular inscription.
You then fill out the applications and within a few weeks granted permission to wed.
If you having Catholic marriage you will also need baptism certificate, mine needed to be translated and approved by the Bishop for the area of Spain we were getting married.
As your marrying a National should be straight forward.
Good luck.