Creating a new article is pretty easy. Go to the library section you would like the new article created and click on the "Add new article" button:
Fill in all the sections and hit save. Your screen will look a little different to the screenshot below as some of the options you see here are only available to the mods.
If you are writing the article in something else such as MS Word and pasting it in, it's best to save all the formatting for here. When you paste in the content, it's important to remove all the associated formatting. This is done to ensure that all the library articles remain consistent in look and feel. It also saves a lot of rework if the forum font/style ever changes. By sticking to the default forum font and style, articles will automatically be upgraded if there are any changes in the future.
You can also select the text and use this button to clear the formatting if you are going back to edit an existing article: