How can you do it? It's easy. The most common process when you design a website is to draw it first on paper or on a graphic design program and then, try to make it as website.
Thanks to Pixel Perfect you cn import your sketch (PNG, JPG, GIF, etc.) and set it on the webstie we are creating to check if resuts are going well.
Pixel Perfect is really good for web designers.
It requires Firebug.