Step by step recipe:

1. 25 min.
   Prepare a blackberry coulis without any sugar: blend 1 kg 300 g blackberries, then strain this purée through a very fine seive, keeping just the juice.
   
   From 1300 g of blackberries you should be left with about 1 kg of juice.
2. 10 min.
   Pour the juice obtained into a high-sided pan and add 800 g caster sugar and the juice of 1 lemon.
   
   Put over high heat and mix with a whisk.
   
   Bring to the boil, whisking frequently, and being careful to ensure that the juice does not catch on the bottom.
3. 3 min.
   As soon as it begins to boil, a foam will form on the top.
   
   Turn down the heat, then skim off this foam and discard it, using a skimmer or, if you don't have one, a large spoon. See below for more on this.
4. 1 min.
   Mix 5 g agar-agar with a teaspoon of sugar to help it mix more easily with the blackberry juice.
5. 1 min.
   Add the agar-agar and sugar mixture to the blackberry juice while whisking briskly.
   
   Turn up the heat and bring back to the boil, whisking continuously to ensure thorough mixing. You can use a hand mixer briefly for this.
6. 4 min.
   As soon as the pan boils, turn off the heat and pot up the jelly. Be careful - it's very hot!
7. 3 min.
   As soon as the jars are filled and sealed, turn them upside down.
   
   This trick forces the air in the jar up through the scalding hot jelly, so sterilising it, which helps the jelly to keep longer without turning mouldy.
8. 
   After a few minutes, turn the jars right way up again and leave to cool before putting them away, preferably in a cool dark place.