1 sugar cube - any kind will do, but use raw sugar if you have a choice
Soak the sugar cube in Angostura bitters in the bottom of the glass.
As an extra, somewhat controversial step, add a tiny bit of seltzer water.
Use a muddler to mash up the sugar cube with the bitters and the seltzer into a syrupy mix.
Add ice cube, the bigger the better.
Pour in 2 oz. of bourbon or rye.
Garnish with a lemon twist.