1. Combine the berries, oranges, and vinegar in a one-quart glass jar with a tight-fitting lid. Using a long-handled spoon, muddle the cranberries and oranges. Cover with the lid and shake. Let it sit for two days at room temperature, shaking at least once a day.

2. After two days, strain the cranberry-orange mixture through a fine-mesh strainer into a saucepan (discard the solids). Add the sugar and heat, stirring, just until the sugar is dissolved.

3. Pour the mixture back into a sterilized glass jar, cover and refrigerate, let cool for at least one hour before using.

4. To serve, fill a glass with ice, add one ounce of the vinegar mixture (or more to taste) and fill the glass with sparkling water. Stir to combine, garnish with an orange slice and serve.