Simple :) Just go to Dashboard > Design > Template Designer.
At Template Designer, choose a template (like shown in the screen capture below). When you have chosen your template, automatically, you will have access to the new customization features. Easy right? :)
And one more thing! I forgot to mention this earlier :D As for the banner, you can go to www.picnik.com. I often use a photo in which I then apply a background colour that I opt *Just use the doodle feature at under Effects*. To make the girl image on my latest header/banner, I used different types of shapes and just arranged them. To write something at the banner, you can use the Text feature. There are lots of fonts to choose from. Good luck! :)