Build in highball, Collins, or double rocks glass. Add ice then Fernet, then Coke to fill (allowing for foam head.) The effervescence from the ice surface will provide the mixing.
Tip: Do not use a single large or long cube, as shown in some videos. A number of small/medium cubes provide more surface area for chilling the drink and more churn from bubbles.
This drink is known by several other names, including Fernet and Coke, Fernando, and Fernandito. The IBA recognizes it as the Fernandito with 50 ml (1 2/3 oz) of Fernet.
The 30/70 Fernet to Coke ratio is the most common according to multiple sources. Some people prefer more Fernet (e.g. 50/50) some even less. Some sources suggest a little lime juice or lime wheel garnish, but this doesn't seem to be the norm.
Fernet Branca has long been a popular beverage in Argentina and most of the world's Fernet Branca is consumed there. The origin date of the cocktail is unclear, but is generally attributed to Córdoba, Argentina. The drink became popular in the 1980's and has become the national cocktail of Argentina.