Adding another section to the sidebar in a Tumblr theme?

Not really sure if this is the right place to ask this, so please move it to the right stackexchange if it isn't.

I'm trying to add another section to the sidebar of this tumblr theme, another separate box just like the "Introduction" box in which I can put any sort of text. Yet when I try to add my new box it either extends the existing box instead of creating a separate one or puts the new box next to the existing box above the area where posts are displayed. Any idea how to make this work?

Basically, it looks like this:

And I want to customize it to make it look like this: