1 1/2 ounces Aperol
1 1/2 ounces sweet vermouth (I recommend Cocchi Vermouth di Torino)
3 dashes Angostura bitters
2 ounces soda

Combine the Aperol, vermouth, and bitters in an ice-filled glass, and top with soda. Stir gently to combine and garnish with orange twist or orange slice.