Put the berry mixture in a saucepan, add the sugar and cherry juice (minus 3 tablespoons), stir everything briefly and then bring to a simmer over high heat.
At the same time, put one tablespoon of cornstarch together with the 3 tablespoons of cherry juice in a bowl and stir together until smooth. Pour this mixture over the simmering berries and stir in. Bring everything to the boil again briefly and then simmer for about a minute.
Then take it off the stove and let it cool down - the red fruit jelly is ready!