If you haven't already, sign up for GitHub by going to the link above and hitting Sign Up at the top-right of the screen (or the menu on smaller devices and mobile), and then navigate to my Wiki repo. You'll see all of the lovely little files that make up this wiki <3
You won't see exactly what I have if you don't use the Gitako extension.
At the top right, select Fork. If you use GitHub and are in an organization, select your name rather than the organization's.
Once that's complete, you're all set to start editing! Head over to the page you wish to edit and click the pencil icon on the row next to Raw | Blame. Before you start editing, however, make sure you read the Contribution Guidelines page.
This will switch to Editor Mode, and then you'll be put at a monospaced screen with lots of syntax for lots of things. Go to the GitHub Markdown syntax page to learn what it all means.
Submitting
If you're ready to submit your changes to the main repo and the wiki, return to the main repository page and check out the Notes on Commits page, send a pull request and you're all good to go!