My husband is American and I am British. We speak very little Spanish but did the paperwork ourselves with the help of the info on this site and another expat site. It's not that complex, however you must have all the correct paper work, notarised and translated.
Your marriage certificate must be an original and "apostilled" - preferably from the UK as this made our process a lot easier than couples who have a US marriage certificate.
We applied at thebeginning of October, however I did not have my marriage certificate apostilled (notarised by the Foreign Office in London, UK) and had to send it there - wait a few weeks and then return to the Immigration office to submit it.
I believe that we were fortunate in that the lady who interviewed us initially was extremely helpful. She basically wrote down exactly what we needed to do and we just translated it when we got back home.
We managed to submit the corrected certificate some time in November and got the appointment for my husband to get fingerprinted and hand in his photos for his residency card three months later. We then had to wait another 45 day to actually pick up the card, however he had a "temporary card", which was more like a paper receipt in the meantime.
I have been through the process in the USA and know about the UK system and at least in our case the Spanish system was the fastest and easiest. Whether or not it is the most efficient is debatable!
You can PM me if you like and I can give you the set of instructions I received when filing.
It does help if you speak Spanish but we just smiled a lot and were very polite. It worked in our favour.