Step 1: Wash and chop fresh rhubarb into 1-inch pieces. Discard the leaves as they are toxic.
Step 2: In a medium saucepan, combine the chopped rhubarb and water. Bring to a boil, then reduce heat and simmer for 20-25 minutes, until rhubarb is very soft and broken down.
Step 3: Place a fine-mesh strainer over a heat-proof bowl. Pour the rhubarb mixture through the strainer, allowing the liquid to drip through. Do not press on the fruit, as this can make the syrup cloudy.
Step 4: Return the strained rhubarb liquid to the clean saucepan. Stir in the granulated sugar until completely dissolved. Bring the mixture to a boil again, then reduce heat and simmer for 5 minutes.
Step 5: If desired, add optional flavorings like a cinnamon stick, vanilla bean, or citrus peel to the hot syrup and let steep as it cools (10-30 minutes). Remove flavorings before storing.
Step 6: Allow the homemade rhubarb simple syrup to cool completely to room temperature. Transfer to a clean, airtight container and store in the refrigerator for up to 1 week, or freeze for up to 3 months.