Here's the recipe I used for suggested quantities, proportionate to the 500 grams of blackberries we had.
Before starting, I put the jam jars in the oven at 170 Celsius to sterilise them and left them until finishing the jam. More info on sterlising jars here.
As the recipe states, I added the blackberries with a bit of water. And then added the sugar and lemon juice. I did not use preserving sugar, but some raw cane granulated sugar that was in the cupboard, I think any granulated sugar will do.
The one trick about jam making is to stop it boiling when it has reached setting point. At this point, it will still appear liquid when hot, but it will settle once it cools.
There are various ways to test the setting point. One simple way is the 'plate test'. Put a plate in the fridge before you start making jam. After about 15 minutes of boiling, take the plate out, place a drop on the plate and return to the fridge for 2 minutes. If the jam reached setting point, the drop of jam will not drip when you tilt the plate. In addition, if you poke it with your finger, it should be slightly wrinkly on top. If the jam is not set, return the plate to the fridge and test again every 5 minutes or so. Here's my plate below at the point when the jam has set, pointing at a drop indicating setting point. And here are some additional instructions for this.
Optimistically enough, I had prepared three jars, but 500 grams of blackberries only made two jars. After setting, leave the jam to cool for 20 minutes or so, then place in the hot jars. Leave to cool in the jars for a while longer, and place the caps on while still warm. And label. Ours was cooked with the London Paralympics in the background on tv.
