Slice the lemons with the peel. Let it steep for 2 - 3 days together with the elderflower and apple juice. I put the juice in the fridge. Strain everything through a cloth, express something.
Measure out 3/4 l at a time and cook 2: 1 with 500 g of preserving sugar according to the instructions on the package.
In my experience, jelly works better when cooked in smaller quantities.
The rest of the mixture can either be drunk, made into syrup or used for my strawberry-elderflower-apple jam.