Injecting Javascript on a live page.

I want to run a piece of javascript on a live page that I don't have backend control of. I know it's possible. I've never done it before though.
I'm in google chrome.

This is what I see. I have no idea what I'm doing:thumbsup:
http://i.imgur.com/0qDTk.png

You can open the "CONSOLE" tab. There you will have some kind of a "command line" where you can enter Javascript code with which you will be able to manipulate the currently active page.