The best jam I've made so far I used 1.3 kilos pieces of plums from our tree, 1 kilo blueberries I bought, and the pulp of 16 feijoas I was given by a friend. And about 2 kilos sugar (yes my friends that's how much sugar there is in jam). And some lemon juice. The feijoias give it a spike of flavour which is really neat.
Recipe: Put all ingredients together (1.3 kilo cut up plums, no stones, 1 kilo blueberries, the pulp of 16 feijoas, 1 kilo sugar, and jam setting pectin). Also I believe I put in a cup of water.
Stir it up, and let it boil until it is really broken down and thick. Pour into sterilized jars (the dishwasher works, and I do keep pouring boiling water into them, and soak lids in boiling water).
Reusing jam jars from the shops is ok for jam! I save all good jars to use for jam.