Use equal amounts of fruit to sugar.

I used 4lb blackberries and 4lb sugar ( makes 9 jars)

Put blackberries into cooking pan. I used a jam pan but a large saucepan will do.

Cover with a small amount of water.

Add juice of a lemon (or 1 tablespoon of bottled lemon juice).
Bring to boil, simmer until soft.
Add sugar, stir.
Leave to boil. Stir from time to time so that the jam doesn’t burn.

Keep testing for set (put a spoonful onto a cold plate – when jam wrinkles to touch it is ready).
Pour into clean sterilised jars.