MyFont_400.font.js. One click to the left you get the main Cufón library that does the actual font-replacement. Link both into your document and then use
Cufon.replace('h1') to, for instance, replace the h1-element of your page.
That’s all. For starters. Cufón also exposes some options for each replacement and a way to easily bind fonts to dynamically created elements through its API. For now I’m just playing around with it, but it really looks promising :-)