There are currently around 6,000 private language schools in Spain, however, the most reputable ones are registered with the FECEI (National Federation of Private Language Schools)
Usually, the minimum requirements are to be a native speaker, have a university degree and some form English language teaching qualification (CELTA, TESOL etc.). Having previous experience and knowledge of at least some basic Spanish would be a bonus.
Meeting candidates in person is very important in Spain, if you are the kind of person that a company is looking for, then shortcomings in requirements may even be overlooked.
The standard number of hours for an English language teacher is 25 hours per week of contact teaching, with an additional 10 hours or so for lesson preparation and marking. The average monthly wage for an English language teaching is around €1,200 before tax. Working as a freelance teacher is another option, which means you can work in more than one school and manage your own time.
Spainwise is a good place to start if considering teaching English in Spain, they also hold annual recruitment fairs and their website provides information on what to expect as an English language teacher in the country.