There are two key routes to open access:
Considered to be the most sustainable method in the long term, and recommended by the Finch report, the gold route involves publishing in a fully open access journal or website.
Subjected to the same peer-review procedures as a traditional journal, the open access journal will usually be available online. Authors may need to pay for their work to be published, although this is very rare as it is often provided for by the research grant. Some institutions even pay these fees out of a central fund to account for the differences between research councils.
Self-archiving in a repository is a concept that is gradually gaining ground. There are risks, as this process relies on researchers uploading their own papers. To counter this, some institutions have made it compulsory for all published work to be added to their respective repositories.
Repositories offer a number of benefits. They increase the availability of some published journal works with restrictions on reprinting or text mining, and may enable work to be propogated across the internet and used for novel applications. Repositories also allow authors to keep track of who is downloading their data.
No matter which route you take, or how far down the road you are, there are a suite of resources available to support you.