I followed the same process I'd used with the blueberries. Combine equal amounts of fruit and sugar and let it sit at room temperature for 2 days, stirring each day. Strain that syrup from the pulp
Add an equal amount of apple cider vinegar to the syrup. At this point, I tasted my shrub. Yikes! Sticky-sweet blueberry syrup had been perfect with the tartness of the apple cider, but tart rhubarb syrup combined with apple cider tasted like the equivalent of being punched. Lesson learned - when using a sweet fruit, an equal amount of sugar to fruit will make an appropriate syrup for shrub. When using a tart fruit, add extra sugar so that you don't end up with something so tart it's undrinkable!
I added a bunch more sugar to the shrub and let it mellow in the refrigerator for a few days. Finally, I had a delicious drinking vinegar. There was a still a small amount of blueberry shrub left, so I poured each so I could compare their colors side by side. Beautiful.
Come winter when our orange tree goes crazy and we have more oranges than we can use, remind me to try making orange shrub, OK? Thanks!