Add two scoops of vanilla ice cream to a tall glass.
Slowly pour root beer over the top until full.
This method prevents overflowing as the ice cream begins to foam.
Notes
To make an adult root beer float, add a shot of dark rum, vanilla vodka or bourbon to the top of the float. Adding a shot (one ounce) of any liquor you love most will turn it into an alcoholic root beer float.
Some recipes call for whipped cream with sprinkles and a maraschino cherry on top.
I like to serve my floats with a straw and a spoon. Much like a milkshake, it's fun to have both options.
Try chocolate ice cream instead of vanilla for a slight variation. Even better, make one of each and compare, choosing a favorite.
If you want to go even more old school, use sparkling water and flavored syrups to create your sodas from scratch.