The following fruits are high in pectin, these can be used singly or combined with low pectin content fruits.

When the mixture is cooked. you must test the mixture for its pectin content. Before you can proceed further.

Take a teaspoon of the cooked liquid and place in a cup to which is added about two teaspoons of methylated spirits. Give the cup a swill to mix the liquids and leave to stand about 5 minutes. Then slowly and carefully pour away the methylated spirits only, leaving the jam liquid in the bottom. If the mixture is a solid mass then the pectin content of your jam mixture is high and a good set is assured, with possibly a shorter setting time. However, if the liquid is more separated then the set will be soft. With any of the above fruit, we would expect a result of a solid clot.
However if the liquid has not combined, the we would think about adding extra pectin, commercial or home made.
If you accept the test is ok for the particular fruit, then you can proceed. Add sugar slowly, stir to ensure that the sugar has completely dissolved (look at the back of the spoon, if you can still see sugar crystals, continue stirring). If using extra taste ingredients such as ginger or stone fruit kernels, these can now be added.

"Once the sugar has been added, no further cooking of the fruit occurs, so make sure that the fruit is well cooked."

The jars are in the oven, the plates in the fridge and you have your thermometer, large plate and skimming spoon ready to hand.
Turn up the heat, until the mixture begins to boil rapidly, this will develop into a 'Rolling Boil'. At no time during this process do you stir the liquid, as the high temperature is causing the pectin and sugar to react. This process will take about 15-20 minutes (the setting time is variable, subject to the amount of pectin in the liquid).
During this boiling stage, you can start skimming off the scum, using the slotted spoon. Some recipes tell you to add butter (size of a walnut), but for clear exhibition jam, removing the scum while it's on the top of the mixture is still the best method. If after boiling there is any scum left, use folded paper towels to soak up the last remnants.
The next crucial stage is judging the setting point. If using a sugar thermometer, insert the thermometer in the mixture and watch the reading climb (jams usually sets at 104-106ºc (220-222ºF) at (ground level). It is best to also use the 'Cold Plate Test' as a final check.
After 10-12 minutes remove from heat. Take a cold plate from the fridge and place a teaspoon of mixture on it. Pop back into fridge for a few minutes, then remove. Holding the plate up to eye level push the edge of the now cold mixture with your finger, if the mixture starts to crinkle as you push it the mixture has set, no further boiling is required.
If only slightly wrinkled, then carry on boiling for the next 5-8 minutes and test again. If no crinkles appear boil for 8-10 minutes.

"Do not boil for longer, as this will not aid setting. Once the set point has been reached, further boiling only degrades the mixture."

Allow the jam to cool for a few minutes. At this time turn off the oven and remove jars from oven. Later you will see that some jams and marmalades require longer cooling times. Jellies are best potted straight away as jelly can start setting in pan. If the jam or jars are too hot, fruit will rise to the top, spoiling the appearance of the finished product.

Beware! Hot Items. Place the pan on a heat resistant table mat, not on your work surface! Using your wire rack for hot jars. Give the jam a stir. Fill your jars using the jam funnel, until the level is 3mm from top. If using metal twist covers put on now. Place hot jars on a table mat, allow to cool overnight.

1. Reboil, using commercial or home make pectin. Disadvantages: will add to unit cost and no guarantee to work.
2. If too soft, use as a sauce over ice cream, fruit or puddings.