Harvest a large quantity of rosellas, to fill your biggest stainless steel saucepan about two-thirds full. Wash well. Cover the fruit, seeds and all, with water and bring to the boil. Simmer gently until soft, and the red colour has faded from the calyx. Strain through a sieve, throw away the fruit and measure the liquid.
Add the liquid back to the saucepan and add a cup of sugar to every cup of liquid (1 litre of juice = 1 kilo of sugar). Heat gently until all the sugar is dissolved, stirring often. Once the sugar is dissolved bring to the boil for one minute.
Take off the heat; add the strained juice of lemons (depending on availability and to taste, approximately 9 lemons to 3 litres of cordial) then stir in citric acid. Bring back very briefly to the boil. Bottle into clean, dry bottles and seal while still hot.