The "leftcolumn" tag starts the left column, and comes first:
The default gutter (spacing) between the left and right column is 10px - however, you can alter this using a "gutter" parameter:
<mainpage-leftcolumn-start gutter="20px" />
This produces a 20px gutter between the left and right columns.
A column is ended by:
Next, the following starts a 300px wide right column:
Again, the 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 a clear to ensure it appears below the columns. You can do this by either making the next element clear, or add the following between the column end and the next line:
I am a 300x250px or a 300x600px box in the right column. Wikia's ad will go here.
I am a sentence in the right column.
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 follow the last main page tag, and have a width of 100%.