Fill an old-fashioned glass with ice.

Pour in the Prosecco. The Prosecco goes first because the Aperol, Select, or whatever you chose contains more sugar, is more dense, and consequently, if poured first, would remain on the bottom and stratify the drink. The bubbles in the Prosecco instead, besides making it lighter, automatically start mixing the ingredients, sparing you from the incumbency. Also, because the less you mix, the more sparkling your drink is.

Pour in the Aperol or else and finally the seltz or sparkling water.