Use the text tab and code up what ever you want, but you will have to use the style, unless you make a css, which will have to be called in the header. Using style will be easier in a post. You can also use a widget to anywhere plugin, widget shortcode, something aloong those lines, and write you coding in to a text widget and then you can drop it anywhere. Just drop the text widget in to the Arbitrary side bar, save the widget, and get the code. Those are the two point five easier options
There have two post mode, when you are posting, you will see HTML and Text Mode. Select HTML mode and write your style coding on it. This will help you to add manual html codes.