Add juice and zests of 4 lemons and 2 limes (which was what I had lying around, 6 lemons would be fine as well).
Add water (a bit more than 1l).
Bring to boil and let cook for a few minutes.
I then ran out of time and let them cool down and stand for a day in a cool location.
Bring to boil again, adding ~1kg sugar when the liquid gets warm (but before it cooks)
Let it cook for a while
Taste. If it's too sweet (which it was for me), add some more lemon juice or (if you're out of lemons like me) just plain lemon acid powder.
Let it cook on low heat, testing if the jelly will set from time to time: take a teaspoon, put it on a cool plate and place it in the fridge for a few minutes. If you like the consistency, turn off the heat.
Fill into sterilized glasses.
The result should "flow" like honey, taste first of lemon, then of sugar, and in the end the hot chilis hit you.