To get the juice from the plums, pit and half the plums. You won't need to peel! Place in large saucepan. Add 1 cup water. Bring to a boil. Cover and simmer 10 minutes. Use strainer or cheesecloth for clearer jelly. I transfer from the main pot to another big bowl, and strain through the juice back into the big pot.

At the same time, get your jam-ming jars ready by boiling a pot of water. Put in the jar lids into the pot and boil for 10 minutes at a constant rolling boil. This is to sanitize the lids! Wash the jars themselves in hot water.  

Slowly mix 1 cup of sugar at a time into the plum juice. Mix well after each cup and check the flavour. Sweeter plums will need less sugar and tart plums will need more! Add 1/2 teaspoon butter (or oil based margarine to keep it vegan)  - this helps keep the jelly clear and free of foam. At this point, bring the big pot to full rolling boil over high heat, stirring constantly. Add fruit pectin and stir right away. Boil the mix for 1 minute, stirring constantly. Remove from heat. Skim off any foam. Fill jars immediately. 

 Wipe the jar rims and threads, as well as the jar sides (I know mine get a little sticky). Spoon out any bubbles in jars (I didn't have much). Cover quickly with lids. Invert jars for 5 minutes, then turn upright. Check seals after 1 hour and you're jelly ready!