It is not just spelling errors that the proofreader needs to fix. There are many other things that they need to do. It is important, then, that those who are seeking proof reading opportunities have the skills necessary to get the job done correctly.
For those who can do this, they will be in demand and have repeated work.
Proofreaders have to read copy and correct any errors in punctuation, grammatics, and also in the spelling. But, most of the time, they also have to ensure that the flow of the piece is correct and that it makes sense. If the author can not convey their message correctly, it is up to the proofreading experts do it for them. For those who do non-English proof reading, it is important that they also know the ins and outs of the language. Many languages have different dialects that are spoken in different areas. For those who speak Spanish in the United States, it is often a dialect of those who speak it in Spain. For the freelance proofreader, it is important to know which type of dialect, if any, should be used.
For those looking to get into this field, they will find opportunities in translations readily available to them. From there, they will find those who need them to write in or edit pieces on their businesses. But by far the most important aspect that they need to have is a good foundation in the language.