Wash the lemons thoroughly and pat dry with a tea towel. Using a peeler, lightly peel the zest off each lemon, being careful not to remove the white part of the pith. If you have some peel with white, you can use a blunt knife to gently scrape it off.
Place the zest into a large mason jar (at least 1L) that has a tightly sealing lid. Pour the vodka over the lemons, seal the jar and then shake the contents around to mix.
Store the jar in a cool, dark place for about 20 days, until the vodka takes on a rich yellow colour. Every few days, give the jar a light shake to move the contents around.
When the vodka is ready, place the water in a saucepan, add the sugar and warm slightly until the sugar has dissolved. Set aside to cool to room temperature.
Once the sugar syrup is completely cool, use a fine sieve to strain the vodka into the water. Then place the lemon peel back into the mason jar, add a splash of water and shake about to release any remaining vodka from the lemons. Strain this into the sugar syrup and press on the peel lightly to remove all the liqud.
Stir the liquid to combine. Place a funnel in a bottle and then using a ladle, carefully transfer the limoncello from the pot into the bottle. Refrigerate and serve chilled.