I'd do a search for Denny Conn's bourbon vanilla porter. It is a well established recipe.
As for how I add things like this to beer: I pour a set amount (8-16 ounces) and I add a small amount in and then I taste. I tend to start as small as possible. I keep adding (and topping off the glass) until I find a good balance. At that point, I'll make my calculation for how much I want in the beer.