I've recently been asked several times by friends about my home-made liqueurs, so here is a rough guide for beginners. I don't pretend to be an authority. In any case, the best way to learn is to follow the Threefold Way which led me to my current enlightenment: experiment, drink, refine.
What is a liqueur? Nothing more complicated than an alcoholic extract (of whatever gives it its flavour), sweetened and thickened with sugar syrup, or perhaps honey or some other sweetener. In preparing a liqueur, no fermentation or distillation is involved: usually an alcoholic drink of relatively inobtrusive flavour is infused with the fruit or whatever else gives the liqueur its taste.
To make a (750ml) bottle of approximately 70 proof (35% alcohol) liqueur:
- Place 1-2 cups roughly chopped fruit or herbs, or a few tablespoons crushed spices, or whatever tickles your fancy in a large jar or other sealable container
- Pour in around 2 cups 100 proof (50% alcohol) vodka, grappa, or other clear spirits, making sure to cover all the fruit or whatever
- Close the jar tightly, and put it aside for a week or two (at room temperature). You might want to label it with the contents and date, especially if you're making several drinks simultaneously, perhaps experimenting with quantities and compositions... Every few days, give the jar a shake, and have a taste to see how things are progressing. Liqueurs from rather delicate ingredients, such as mint leaves, should probably be left around for no more than a week, as the contents may "spoil", turning an unsightly brown and losing their delicate aroma; most ingredients will benefit from a two-week infusion, and won't suffer from being left in for twice that.
- Strain the liquid into another container (a regular strainer should do, or use a paper towel, paper filter or cheesecloth, if you like
- Measure how much liquid you have. Depending on what you were extracting, you might now have more than 2 cups (because the fruit lost some liquid), or less (because something absorbed liquid). For the following quantities I'm assuming you still have 2 cups, but if not, remember that what you're really tracking is alcohol content, which hasn't changed (in other words, however much you now have, if you top it up to a 3-cup bottle you will still have a 67 proof drink).
- Mix in about 1 cup simple syrup (prepared by stirring together sugar and water at a ratio of 2:1 over a low heat, and cooling); do this gradually, tasting in between or first mixing a sample, to make sure you get the sweetness and alcohol content you like, and use water, uninfused spirits, or sweeter syrup to make corrections
- Bottle the resulting liqueur, and again put it aside to rest for a week or two -- the flavour and texture will continue to improve, so it's worth waiting
- Serve in liqueur glasses, or perhaps shot glasses, and enjoy!
Good fruit to use are peaches or nectarines, cherries, any berries, melons, apples, apricots, mango, orange or lemon zest, and probably a long, long list. Crushing the pits first (especially of cherries, apricots, and the like) will add a more complex almondy flavour (yes, that flavour has to do with cyanide, just as it does in almonds, but unless you intend to imbibe several bottles of the resulting liqueur at one sitting, you're pretty far from the dangerous dose). If the skins have an interesting colour and/or flavour, consider whether you want your fruit peeled or not: peach liqueur sans skin is yellowish, or whatever colour the peach flesh is; with skins it turns pinkish. Anything you'd use to make tea or a tisane (mint leaves, sage, etc.) will probably make a good liqueur, and come to mention it tea leaves and various tea blends make for an interesting (if rather bitter) concoction. Spices, especially dried spices, should probably be used in smaller quantities: a few tablespoons per bottle. Interesting choices are a bit of fresh ginger, a star aniseed, a tablespoon or two of crushed peppercorns, allspice, coriander seeds, cumin, or cardamon, some roughly grated or slivered nutmeg, a small stick or two of cinnamon, or a few cloves. Powdered spices will be harder to filter out later, and might be less flavourful.
Instead of the sugar syrup, you might try honey (especially with a brown/yellow spice liqueur), or an artificial sweetener (mixed with a suitable amount of water) if you can't or won't take sugar. You can also make invert sugar syrup by adding to your syrup a spoonful of lemon juice or cream of tartar, or some other (neutrally- or compatibly-flavoured) edible acid you have on hand. This gives a somewhat smoother effect. I'm told glycerin is sometimes used to make a thicker drink, but haven't tried this yet.
A stronger base drink (e.g. 120 proof vodka which you might be able to get hold of) will extract more flavour faster, and a weaker one (such as the more widely available 80 proof drinks) will do a less thorough job, as well as possibly forcing you to water down the liqueur below the alcohol content you'd like, when adding the syrup. Cheap alcohol (perhaps filtered through your local Brita filter, if you get a chance) gives almost identical results to more expensive choices: possibly the fruit etc. help pick up the impurities in the lower-grade stuff. A drink with more character than your average clear spirits is sometimes an interesting base: cheap brandy is miraculously transformed with the aid of some citrus zest.
Finally, you can of course make the liqueur sweeter, stronger, more heavily flavoured, and so on. The arithmetic falls short of the proverbial rocket science: add another cup of water, and you get a 50 proof liqueur; make a sweeter syrup, and you can drop in the same amount of sugar, without going much below 80 proof.
The best advice, as I mentioned before, is: experiment. Cheers.