The fruit you use determines the sugar you need. For high pectin fruit like raspberries and strawberries use jam sugar, for low pectin fruit like blackcurrant or plum use preserving sugar.
Use equal amounts of fruit and sugar. That's all you need to remember.
Simmer your fruit to mush it down. After a couple of minutes add the sugar, stir and let it all melt.
When stirred in, crank up the heat to a full boil. Boil for approx 5-7 mins or until it reaches 105 degrees C (221F) which is the setting point.
To test, dip a spoon in. If it wrinkles after 30 seconds it's ready.
Pour through a sieve into jam jars and put lids on to cool.