CAREFULLY to avoid explosive head formation: Pour Beer glass half full of favorite rootbeer and top off with Guinness.
I prefer Henrys for the rootbeer (cold) and the Guinness warm and flat. I have used other beers and for me stouter is better. Porter works. You may vary the 50/50 proportions to taste. One of the most charming traits of this drink is the disgusted looks you get when you order it.