In a large pot over high heat, bring the water, sugar, ginger, and lemon juice to a boil.

Turn heat to low and simmer for at least 10 minutes; simmer it for up to 30 minutes if you want a stronger, spicier ginger flavour.

Remove the pot from the heat and let cool to lukewarm. Strain through a fine sieve or cheesecloth to remove the ginger pieces.

Pour the ginger water into a plastic 2 L bottle, leaving about 5-7 centimetres of headspace at the top.

Add the yeast and seal the bottle. Let the bottle sit at room temperature for 24 hours. At this point the bottle will be pressurized with air; open the lid to release some of the gases. Reseal and store in the fridge for another 8 hours or overnight.

You can either drink it at this point, or siphon off the ginger beer from the yeasty sediment at the bottom of the bottle. To do this, keep the brewing bottle at a higher level than the clean bottle to which you will be siphoning the beer. Take a plastic tube and insert one end into the brewing bottle, making sure not to disturb the sediment. Have the clean bottle at a lower level, ready to go. Give the free end of the plastic tube a quick suck to get the ginger beer moving, and quickly place the free end of the tube into the clean bottle.

Store in the fridge.