1. Take about 30 FRESHLY PICKED limes when they're just ripe (ie, before the skin has started to yellow) and at their most aromatic. If they're not aromatic (or worse, they're supermarket limes) it doesn't work well. Test them by scratching the skin with a fingernail and sniffing. If they don't smell wonderful, don't bother – this is an all or nothing recipe.
2. Rinse the limes and remove any blemishes. With a potato peeler, carefully shave the green zest off, trying to avoid the white pith (it will make the liqueur bitter).
3. Put the zest in a large jar with about 250 g of sugar (more or less according to how sweet you like the liqueur).
4. Pour in a litre of good quality vodka, cap and shake until sugar is dissolved.
5. You can give the liqueur a bit more colour and fragrance by adding a few freshly picked lime leaves to the jar, if the leaves are really aromatic.
6. Cap the jar and leave in a cool dry place for a minimum of six weeks (3 months is better; longer than that doesn't make any difference).
6. Strain off the zest, bottle the pale green-gold liqueur and allow it to age for a couple of months, or longer. It lasts for years but is best drunk within a year or two.
7. Before use, put the bottle in the freezer for several hours (it won't freeze due to the alcohol content). Drink ice cold, preferably from a chilled brandy balloon, in good company. Or listening to Hayley Westenra sing Amazing Grace, http://www.youtube.com/watch?v=BOrlCYp8TgU.
Theoretically this should work for other citrus fruits but I haven't had nearly as much success with oranges or lemons – I think because the oils in the peel weren't strong or fragrant enough. I'm going to try it with Seville oranges, when my tree is big enough.