It seems like such an easy and obvious drink that it barely constitutes a recipe, but the Scotch & Soda has a purpose and it is a popular way for many drinkers to enjoy their favorite Scotch. The concept here is to water down a Scotch without destroying the flavors of the whiskey, making the drink last a little longer, go down a little smoother, and make it a little more refreshing. It is a great way to enjoy Scotch on warmer days.
Just as adding non-carbonated water to whiskey opens up the flavor, the club soda will do the same and add effervescence as well. I tend to stick with medium-grade blended Scotches with this drink, something like Johnny Walker Black. Also, if you have the option, choose a quality club soda like Fever Tree. These have a cleaner profile than the average soda and are really nice with a decent Scotch.
The amount of soda you use will really be up to you. Some people prefer to fill a highball all the way, while others may opt for a lowball glass and a splash or two of soda. I do recommend using an ice ball or large cube because they melt, therefore dilute, slower than cubes.