Use a cocktail tumbler, sometimes called an old-fashioned glass. Do not use a pint glass such as one might use for drinking water or beer.
First, fill the glass loosely with ice cubes, maybe two-thirds full.
Then, pour in about one-half of the glass with non-flavored vodka. After that, add in a little bit less than half of that amount of Kahlúa. Stir ice, vodka, and Kahlúa together until well-mixed.
To finish, fill up the rest of the glass with cream or milk, poured in slowly to float on top of the mixture. Do not stir again; instead allow the cream to gradually mingle with the liquor on its own terms. Abide.