1Juice the lemons and put the skins in a saucepan. Pour over the water and quickly bring to the boil, stirring/turning over continually. You want as much of the zest flavour and a tiny bit of pithiness.
2Strain immediately and gently squeeze the skins when they're cool enough. Return the liquid to the pan and gently heat. Add the sugar, stir until dissolved and add the lemon juice.
3Strain again and hot fill into a sterile bottle. Keep the surplus in a jug in the fridge.
4The bottle will keep for a month or so in the fridge - if you want to keep it for longer, pasteurise to 80°C in a water bath - although finding a vessel big enough to pasteurise whole bottles is tricky and reheating does result in losing some of the zestiness. You can also strain into plastic bottles and freeze.
5You can adjust the method to use other fruit as well - gooseberries for example work well with an equal amount of lemons.