Set up steam juicer as per manufacturer directions, which usually means water in the bottom pot and rhubarb in the top pot. Boil until the steam releases the juice. Make sure the bottom pot doesn't run dry.
How to make juice on the stove top
Boil two pounds of rhubarb in four cups of water until the rhubarb completely falls apart.
With a jelly bag or cheesecloth-lined strainer suspended over a large bowl, strain the rhubarb. Let it sit in the jelly bag or strainer for a couple hours to allow the juice to fully drain from the rhubarb.
How to make and can rhubarb jelly
Pour rhubarb juice into a large pot and bring to a boil.
Add the pectin, return to a boil, and boil for one minute.
Add the sugar, stir to dissolve, return to a boil, and boil for one minute.
Remove jelly from heat and immediately ladle into jars, leaving a ¼ inch headspace.
Wipe jar rims with a paper towel wetted with white vinegar and attach two-part lids to fingertip tightness.
Put in water bath canner and cover with hot water until jars are submerged by at least an inch of water. Process in canner for 10 minutes, starting the timer when the canner reaches a full boil.