Five parts mixer to 1 part shrub--a sweet-tart infusion of vinegar and fruit--is a good place to start for this spritely drink per Mary Karlin, author of Mastering Fermentation, but feel free to play with the ratio so the flavor is milder or more bracing.
Total time: 5 Minutes
Pour shrub into a tall glass. Top up with sparkling water, and stir. Taste and adjust proportions if you like. Garnish with pineapple spear or grapefruit wedge.