What are the steps to obtain a long stay O-A visa for Thailand from Spain?

Okay, Serge Bournac posted in another (now closed) thread that he wants to get a long stay O-A visa

Your options are

Go to the thai consulate website in your home country, look at the requirements for a year-long, multi-entry Non-Immigrant Type O-A or Type O-X visa. Meet the requirements, apply and get the visa.

These are the requirements to get an O-A visa from the thai consulate in Madrid Spain. (I just used them as an example seeing as your profile says you live in Spain).