I had sticks and stacks of rhubarb after we sorted out the rhubarb patch at the allotment. I froze some but I made some gorgeous rhubarb jam which is very frugal despite the jam sugar because the rhubarb itself was free so I'm sharing it with a clear conscience.
Trim, wipe and chop the rhubarb. Weigh it (important so don't forget)
Stew the rhubarb in a little water in a large pan until soft. I'm lucky enough to have a maslin pan but any large pan would do.
Remove from heat and add the same weight of jam sugar as the rhubarb weighed. Add a good squeeze of lemon juice (from a bottle is fine). Stir well and leave for the sugar to dissolve into the fruit.
While that's doing or when you're ready, whichever is later, thoroughly wash your jam jars and lids. Put the jars on an oven tray in a cool oven (about 80 to 100 C fan) and place two saucers in the fridge to check for set.
Put the fruit/sugar on a lot heat , stirring regularly, until it starts bubbling. Turn up the heat and cook it (still stirring) for about five minutes and then take it off the heat and check for set. If it hasn't reached setting point, boil for another five minutes and check again. Mine set first time, despite the low pectin.
Rhubarb is low in pectin, but the jam sugar and the lemon juice should sort that one out.
This link is the best article on setting point that I have seen - point 4, the wrinkle test, is what I use and it always works.
Once set, ladle into the warm jars, screw on the clean lids and allow to cool before labelling.
Basically, this is how you make all jam although if the fruit is high pectin I use ordinary granulated (a lot cheaper), not jam sugar.