Combine all ingredients, gently muddle the mint to express its oils, then remove the mint. Shake the rest with ice and serve straight up.
I made the shrub by chopping up a ripe plum into cubes and tossing it into a jar with an equal amount of sugar. Once all of the liquid was extracted after a few days, I removed the plum pieces, and I added an equal amount of apple vinegar. My ratio was 1/2 cup plum, 1/2 cup sugar, 1/2 cup vinegar.