The problem with light beer is that it is very difficult to do with extract. The fermentability of wort can be controlled when doing all grain brewing through controlling the mash temperatures, when you work with extract you are stuck with the fermentability they give you in the can or bag.
That being said some people have had success (Myself included) making light beer by throwin crushed up beano tablets in the beer as it ferments. Beano contains an enzyme that breaks down the complex carbohydrtes into fermentable sugars, however there are some drawbacks as you can't stop the enzymes once they have begun working leaving a very dry watery beer.
If you wnt to take this option just brew a standard sam adams clone, the recipe is available in the clonebrews book and online, then add beano during fermentation.