@Overanddun
The start is a bit vague but I promise it works!
I weigh my blackberries, and then add roughly (doesn’t need to be exact) the same amount of baking apples. So, if I have 2kgs of berries then the same in apples - peel, cores and all. The key is to get a good balance of high pectin fruit and that’s in the apples and especially peel. Basically double up fruit with baking apples.
Put the fruit and quartered apples in a large pan, cover up to three quarters, and bring to the boil, then simmer until mushy. Then cool.
Then, you need to strain all this - I put a colander into a large bowl, then line the colander with a clean baby muslin, and pour the mixture in. Tie the muslin together and hang it up to strain into the bowl - I do it from a wall mounted cupboard door handle - my mum used a Barnes Wallis type chair configuration! Let it drip overnight.
The following day, give the bag a final squeeze with your hands, and discard the fruit in the muslin. Measure the juice and return to a large clean pan. For every pint of juice, you add 1lb of sugar (preferably jam sugar). Boil up until the sugar dissolves and the temp reaches 105c - I use a sugar thermometer but you can also check it with cold plates in the freezer. Pour into sterilised jars, and cover with wax paper circles and then seal.
If you are using metric, the measurements are 450g sugar for every 570 ml of juice.