Make sure that you install Firebug and then drag the "jQuerify" bookmarklet link to your bookmark toolbar. The bookmarklet will enable you to inject jQuery into a page should it not already have it.
Once you have all of that set up simply go to a page, and open the Firebug console. Open the command-line (icon pictured below).
You'll then see the command-line at the bottom (pictured below).