Zest and juice the lemons, removing all the pips.
Pour a little of the vodka into a pan, add the sugar and heat gently until dissolved.
Take the pan off the heat and allow to cool slightly before adding the lemon zest and juice. Stir, add the rest of the alcohol and allow to cool completely.
Pour into a sterile bottle. You will have slightly more limoncello than the original 750ml bottle of vodka.
Allow to sit for 24 hours before straining. Store in the fridge and serve very chilled.