I took two lemons, zested and squeezed them, added some water and some sugar and boiled for a bit. Then I tasted and it seemed a bit tart so I put some more sugar in. Finally I added piles of chopped mint and boiled a bit more. At that point it set if I dripped it onto a cold plate, so I strained out the mint and zest and stuck it in the fridge to congeal. It was good.
Thing is though, I have no idea if I could have made a larger quantity of less acidic jelly if I'd put more water in, or if it would then just have been a syrup. Or if I could have got away with one lemon. I have a vague feeling that this has something to do with the pectin content of the lemons, but how one estimates that, I do not know.
Does anyone know about the theory behind jam and jelly recipes? I'm OK with making stuff from a recipe, or even improvising based on a recipe - but would like to know more about how the gelling bit works so I can be more freeform about my jellies in future! Google is not being helpful today: it gives me recipes, not an understanding of the principles behind them.