My wife (who has no interest in boardgames) writes:
This would probably work for gin as well, if you like juniper which I don’t.
First, and important: do not make this using a narrow-necked bottle. Getting the blackberries out again without making an astounding amount of mess is not possible. Find a wide-necked jar that is large enough to hold the amount of vodka you want, plus half as much again for the blackberries and sugar. I use large old pickled onion jars from a pub, which were free. Kilner jars would do.
Second, do not use good vodka. It doesn’t make the end result taste better. Because the nearest shop here is Tesco, I use their cheap own-brand “grain vodka”, and I buy one-and-a-half-litre bottles because that is cheaper per volume than one-litre bottles are. It keeps. It keeps best at the back of a kitchen cupboard where nobody notices it. I expect other supermarkets sell the same stuff with different own-brand labels. I am told the very cheapest gin makes nasty fruit gins, so one step up from the very cheapest is advised for gin.
Keep the original bottle! You will be using it, re-labelled, to put the blackberry vodka into in a few months’ time. (Small bottles are also fine. Glass bottled-water ones from the pub are free.)
Pick blackberries. Pick lots, because they freeze well and you could do it again in February if you have frozen blackberries available. I do raspberry using 500g frozen raspberries to 1.5 litres vodka, because it’s way cheaper than using fresh ones and you can make it out of season.
Put the vodka into the jar and add blackberries until the jar is nearly full – call it 500-800g, or just drop them in until you have enough. Add three dessert spoonfuls granulated white sugar. Remember you can always add more later, but you can’t add less. All the recipes on the internet have too much sugar for my taste, so you end up with a syrop.
Label it with the date and leave it in the way somewhere. For the first couple of days swirl it around when you happen to remember, to dissolve the sugar. Once the sugar is dissolved, put the jar out of the way and forget it for a couple of months or so. (Traditionally the autumn’s blackberries make the Christmas blackberry vodka, which gives a rough time of between two and four months; twelve weeks is generally recommended.)
When ten weeks or so have gone by and you happen to remember it, have it out and taste using a teaspoon. At this point if it tastes as you like, bravo: you have made blackberry vodka. If you want more sweetness add sugar. if you want more blackberry, leave it for a bit longer. To the best of my knowledge, another month of steeping because I forgot about it has never done any harm to any blackberry, raspberry, pear, plum, damson, sloe or dried fig vodka I have made.
When you reckon it’s as ready as you are prepared to wait for, and it is a nice deep colour whereas the blackberries have gone almost white, first use a sieve to get the blackberries out of it: put the sieve over a bowl or saucepan large enough to take the vodka and have some freeboard for dripping, and leave it to drain the blackberries. (I am told you can eat these or make a boozy crumble with them, but I never seem to do it: the anaemic colour puts me off.) When they are drained, put a funnel in the top of the jar it all came out of, fold a piece of kitchen roll into quarters, and fit the kitchen roll as a filter into the funnel. Gradually strain the vodka+blackberry into the jar. Then repeat the process from the jar into a bottle. You did keep the original bottle, didn’t you? If you want to give some away, you can decant it further and make fancy labels for smaller bottles.
That is It. Really simple, lots of mystique around it. Everyone has their own way of doing it, and exact quantities and timings, but this way seems to produce vodkas that make people happy.