The essence of simplicity, all you need to do to make one cup of rhubarb syrup is chop 2 cups of rhubarb, put it in a pan with 1/2 cup water and from ¼ to ½ cup sugar, and heat over low heat until the rhubarb has cooked down. Press it through a sieve, and voila!
You can control the sweetness level by adding the sugar in stages. Start with ¼ cup. Taste after the rhubarb is cooked down and adjust to your liking.