|Japanese plums (ume) are in season right now; shochu and vodka are always in season|
And if you can make umeshu (often mistakenly called "plum wine;" shu actually means alcohol), you can make liqueur from practically any fruit, and you don't have to go to any special Asian supermarket to do it. But I'll start with the umeshu recipe, which is how I started, and then tell you how to riff off it.
The hard, bitter, acidic plums (ume) needed for this sweet drink are in season right now. Nijiya sells them as well as bags of rock sugar, and has a sheet of instructions there for you to take notes. The only thing Japanese supermarkets do better is selling the container right there also; you'll have to find a large sealable glass jar somewhere else.
But making the stuff couldn't be easier. Here's how.
1) Soak 1 kg (2.2 pounds) plums in cold water for about 2 hours to remove some of the bitterness. Note that these plums are not edible if unprocessed.
2) Remove the stems (sorry, whole-cluster Pinot Noir fans).
3) Wash and sterilize -- I use boiling water -- a large glass jar. Let it cool.
4) Put the plums on the bottom. Add rock sugar directly on top of them. The recipe calls for 700 g (about 25 oz) of sugar but I find that a bit too sweet. But you do need sugar; otherwise these plums will pucker you like raw lemons. I'd go with at least 500 g (about 18 oz) on your first batch. Because after buying the glass jar, you're going to want to try this again.
5) Pour in 1.8 liters of shochu. At least that's what I did in Japan. But frankly, in this country, I'd use vodka. They're both distilled clear liquor and the only real difference is the alcohol percentage; shochu sold in big bottles is generally watered down to 35% alcohol, whereas vodka in that size is 40%. You're making liqueur, and the point is to be able to drink it over the rocks, so I would add some water to the vodka to bring the alcohol percentage down a little. If you buy a 1.75 liter of vodka, add about .175 liter -- one/tenth the amount, or about 6 ounces -- of water. Just pour in the vodka and the water. It'll mix on its own.
6) Seal the jar and store it in a dark, cool place.
That's it. You don't need to stir it or anything. You can start drinking it within about 2 months, but it's much better after about 4 months. Just leave it in the original jar with the ume (or other fruit; see below) while you drink it up. You can put an ume in a glass of umeshu on the rocks for character, but they never do taste as good eaten as drunk.
As for riffing ... I made all sorts of shu after my initial forays with ume. My most successful was Philippine mango; my wife still talks about it. Most fruits are sweeter than ume so I used gradually less sugar, but it is possible to use too little; dry blueberry shu didn't actually turn out to be delightful. You can also do savory shu; my most successful was with shiso (a leafy herb often found with yakitori.) Once you have the jar, your only investment is the fruit, a 1.75 liter of vodka, some rock sugar and 2-4 months waiting time. That's the hardest part.