Make sloe juice from 4-5 kg sloes in a steam juicer. (Recipe in my profile. For the jelly, juice the sloes without sugar.)
Mix 60 g of Gelfix with 6 tablespoons of sugar, then stir in the juice. As soon as everything is boiling, add the remaining sugar, bring to the boil again while stirring and cook for at least 3 minutes.
If necessary, skim, fill into the prepared jars, close with screw lids, turn over and let stand on the lid for about 5 minutes.