The phrase Green Open Access refers to an author sharing a copy of their article via a repository or website after publishing in a traditional journal.
In many cases, the publisher of the journal will have language in the author contract that states what version of the article can be shared as well as how soon after publication. For example, sharing a pre-print version of an article is generally acceptable at any time, while the accepted manuscript (the version that will actually be published) will most likely have some sort of embargo period before you're allowed to share it.
It is important to carefully read contracts between you and the specific journal you are publishing in. Even journals published by the same company might have differening contractual rules regarding Green OA.
Gold Open Access is when the final, published version of an article is available, without restrictions, to the public.
Journals that provide gold open access can be funded in a variety of ways, including: being subsidized by an institution or government; charging the author a fee to cover the costs of publishing; advertising revenue; or a freemium model.
You've just written something and want to make it openly available, what are the steps?