This step is very straightforward. Similar to achievements, if you want to submit a guide to WGG, there first must be a Guide List. On the Game Home Page, scroll to the bottom and click "Add Guide List" (if a guide list doesn't already exist).
On the bottom of the guides list, there is a button that says "Add Guide". This is going to be the homepage for your guide. Title your guide appropriately, and add whatever you want to this page. If you want to add your entire guide to this one page, go for it.
If you've created wiki-sub pages on this site, you already know how this works. From the Guide Home Page you can create Guide Sub Pages. These Guide Sub Pages will be a level below the main page. You can also create a Guide Sub Page from a Guide Sub Page, letting you structure your guide like a book.
Take a look at my Call of Duty: Black Ops Veteran Difficulty Walkthrough as an example of a completed guide.
As always, I appreciate feedback, and if something doesn't make sense, don't hesitate to ask.