Place the chopped rhubarb and water in a saucepan. Bring the mixture to a hard boil and then turn the heat down to a simmer.
Simmer rhubarb and water for about 20 minutes, until the rhubarb has completely fallen apart.
Pour the rhubarb mixture into a jelly bag (or cheesecloth-lined fine mesh strainer). Allow the rhubarb juice to drain for at least 2 hours, or until you've collected 4 cups of juice.
Pour the rhubarb juice back into a saucepan and bring it to a boil.
For a full sugar recipe, add the pectin and bring to a boil for one minute.
Add the sugar and return to a boil for 1 minute.
Turn off the heat and then pour the finished rhubarb jelly into prepared jars, leaving 1/4 inch headspace.
Store the preserve in the refrigerator (or freezer) for immediate use, or process in a water bath canner for 10 minutes for a shelf-stable rhubarb jelly.
Notes
For a low-sugar rhubarb jelly, use Sure Jel Low Sugar pectin, and then reduce the sugar to your taste, using as little as 1/4 cup.