Depending on your cup size the amount of ice cream and root beer will vary. For a small mason jar I used 1 ½ scoops of vanilla ice cream (Vanilla Bean being my favorite).
Slowly pour Root Beer to the top. You will have to drink a few of the fizzies to get it to the top.
Using a spoon stir the root beer and ice cream together. You don't have to completely melt the ice cream just mix it around.
For the adult version: Following the same steps as above but add 2 ounces of vanilla vodka.
Notes
Always add your ice cream first! It is way easier to control the fizzies while pouring your root beer over the ice cream rather than vice versa.