One way is to publish an article in a subscription–based journal and then self-archive it in a repository where it can be accessed for free. The repository can be an institutional repository such as DR-NTU or subject-based ones such as arXiv and RePEc. This is known as Green Publishing or Green Model. The repository allows search engines such as Google to crawl their content so the articles are accessible on the Internet.
Most academic publishers allow their authors to self-archive their publications in this way, though most only allow archiving of the “accepted version” of the article. This version is the final manuscript that has been peer reviewed and changes have been made, but not the final formatted version published by the journal. Some publishers require a 6-12 months embargo, before the article can be made available in the open repository. NTU supports this route.
To use the Green Model to make your paper open accessible, rather than depending on the publisher’s self-archiving policy to support green model, your own self, as the author of the paper, can act in a more proactive way. It is common that many publishers’ publishing agreements have asked authors to surrender more of their rights than necessary for publication. By strategically managing the contract negotiation conversation with the publisher, you will be able to place yourself in a good bargaining position to retain copyright to your articles for open access.
The other way authors can make their articles open access is by publishing it in either a pure open access journal, or a hybrid open access journal. This is known as Gold Publishing or Gold Model.
A pure open access journal does not charge usage fee from the users, however, many do charge an article processing charge (APC) from the author before they proceed to publish the article.
A hybrid open access journal is a journal whose business model is at least partially based on subscriptions, and only provide Gold open access for those individual articles for which their authors (or their author's institution or funder) pay an APC.
The APC varies but can go as high as US$3000-4000. Some publishers say they will waive this fee in certain circumstances. If you wish to adopt this model, you need to use your own source of funding (e.g. research funds, school funds, etc.) to pay for the APC.