In a pan, boil 2 litres of your water and then add 3kg of caster sugar. This stops the sugar from burning on the bottom.

Stir until dissolved completely.

Add in your remaining 3 litres of water, as cold as you can get it and pour the whole lot into your homebrew bucket.

Add your sachets of champagne yeast and stir in. 

When the temperature of your syrup mixture has reached below 25°C add your lemons and your bag of elderflower.

Using a wooden spoon, stir your lemons and elderflower thoroughly into the mixture. The temperature is important as you want flavour from the elderflower, not a brown mush. Important: Follow the instructions with your hydrometer and take a gravity reading now. This will enable you to determine the alcohol abv on the second reading. 

Keep your container in a warm place; champagne yeast is good for temperature fluctuations, but does need a warm place that's not cold overnight. The garage is no good, but a conservatory is perfect. 

Monitor and check for signs of life from the champagne yeast. 

After seven days, strain the elderflower and lemons from the container, and pour the remaining liquid into your empty clean pop bottles. Do not fill more than two-thirds full; this allows for the second fermentation and the gasses produced to make fizz.

Again, store in a warm place.

When the gasses have made the bottles hard, undo one and take your second hydrometer reading. It should be between 11% - 13% ABV. If you want it stronger, leave it a couple of more days. 

When you have the required ABV place immediately in a fridge that is below 4°C. Cold kills fermentation and will stop the yeast from fermenting any further. 

You now have Elderflower Champagne. Enjoy!