The "leftcolumn" tag starts the 670px wide left column, and comes first:
The left column is ended by:
Next, the following starts a 330px wide right column:
The right column is ended by:
After the end of the rightcolumn tag you can place anything you wish, and make them 100% page width. (Please note this is recommended against, especially on shorter pages.) However, you will need to add the following to ensure it appears below the columns:
I am a 330x250px or a 330x600px box in the right column. Wikia's ad will go here.
I am a very long sentence in a box with a height of 500px in the right column. Any sort of wiki content could go here -- a featured article or picture, a list of popular categories, an embedded widget, a video or poll or calendar -- anything you like.
I am a sentence in a full-width box beneath both columns