One of the things I sell on my website is Text in sticker format for the application to a cars rear window for example, a website address. On the Purchase page, the customer would enter the text in a text box and then select the color and font from drop downs.

Now on this purchase page, I want to have a link that will say "SEE HOW YOUR GRAPHIC WILL LOOK" and when clicked, another window will open where they can enter their text, color & font and have their idea displayed exactly how they want so they can see before they buy.
For an example, the Font website DaFont uses this same method.

Any ideas how I could implement it?? Any help is appreciated.