Authors do not have to pay any charges for 'green' open access.
Deposit your output in a repository which is open and searchable, with a licence which permits readers to save their own copy.
Check the terms of your Agreement to Publish: can you deposit the published version, or your accepted manuscript? (sometimes called a 'post-print' or 'uncorrected proof'). Contact your publisher if you're unsure, or refer to SHERPA RoMEO, an authoritative database of journal policies maintained by Jisc.
Your publisher may require an embargo on the repository copy for a few months after publication.
Some scholarly disciplines have created their own repositories, and many scholarly institutions also maintain a repository for their own researchers' outputs: see the adjacent column.
Sharing your output via a scholarly network such as ResearchGate, Academia.edu or LinkedIn does not count as 'green' open access for REF or funder compliance, because these platforms don't commit to preserve the outputs or provide a stable location. Their terms of service normally hold the author liable for any breach of publisher terms.