What is the best way to make a strip of blonde with the product sun-in?

I have long, dark brown hair and I'm wondering what is the easiest, and most successful technique to build up a color using the product sun-in? And without getting the lightener all over- All I want is a 'strip' of blonde...and by the way I know that I would have to build up the color to get to blonde:P *And please answer if you have already tried this product:) Thanks

Cut a hole in a shower cap and pull the hair you want changed through the hole.