In a medium saucepan over medium heat, bring sliced rhubarb and water to a simmer. Reduce the heat to medium low and cook for 20 minutes. The liquid will reduce and the rhubarb will break up into a pulp.
2 stalks rhubarb, 2 cups water
Place a fine-mesh strainer over a large bowl and strain the rhubarb mixture into the bowl.
Measure 1 cup of hot liquid. Stir in 1 cup sugar.
1 cup granulated sugar
Let cool completely, then funnel into a jar or bottle. Use immediately or store in the fridge for up to one month.
Notes
When selecting rhubarb, keep in mind that red rhubarb is sweeter and green rhubarb is more sour. Aim for mostly red or a mixture of red and green for the best flavor.
Do not use any of the rhubarb leaves — only the red and green stalks.
Strain out the rhubarb from the liquid well, possibly even twice, to get all the solids out of the syrup.
Add more sugar (either a 1.5:1 or 2:1 ratio) to make a thicker syrup.