Remove pits and slice fruit into pieces. Put them all into a pot on the stovetop over medium high heat with the water. Once it starts turn heat down to medium to boil and put the lid on. Heat for 10 minutes, then remove.
Use a cheesecloth to drain the juice from the fruit with your pot or a bowl underneath. Squeeze tightly and let it sit and drip for a good 20 minutes until you have all the juice you can, then discard the remaining fruit chunks.
At this point you should have at least 5 cups of plum juice minimum. Pour this into your pot over medium heat with the butter and pectin. Bring to a rapid boil.
Stir and add sugar, continue to stir and boil for one more full minute. Then remove pot from heat. If there is any foam on the top scoop that up and discard.
Pour into glass jars and allow to cool, put lids on and store in fridge. Or can properly to store for longer.