Macerate the Rhubarb
In a large bowl or jar, combine the chopped rhubarb and sugar.
Stir well until coated.
Cover and refrigerate for 24 hours. The sugar will draw out the rhubarb juices and create a bright pink syrup.
Strain
Pour the mixture through a fine-mesh sieve into a saucepan, pressing gently on the fruit to extract all the juice.
Discard the solids (or save them to spoon over yogurt or oatmeal).
Heat
Add the apple cider vinegar and champagne vinegar to the saucepan.
If using any optional flavorings, add them now.
Bring to a gentle simmer over medium heat for 5–10 minutes. Do not boil.
Remove from the heat and let cool completely.
If you added spices or herbs, strain them out before bottling.
Bottle
Pour the shrub into a clean glass bottle or jar.
Refrigerate for at least 24 hours before using to allow the flavors to mellow.