This is something I've done for some time, but became serious about it when I first started considering options for building a retirement income. I started proofreading for an online service called My History Digest which publishes stories in various languages. Stories that have been written in a language other than English are then translated by someone whose first language is not English. I receive these and clean them up for publication.
My goal is to make them easy to read and understand without radically changing the style and presentation of the original author.
I have also been proofreading archaelogy articles for a Hungarian archaelogist. In late 2015 I was invited to become a member of the editorial board of My History Digest.
I would welcome the opportunity to take on additional proofreading projects.