Step 1. Pick lots of Sloe berries! Depending on how much liqueur you're planning on making, make sure you have enough berries to be just covered with the schnapps. According to tradition, you should pick the berries after the first frost of the year, since the berries become sweeter after being frozen. But you can just as well leave them in the freezer over night.
Step 2 Pour the alcohol over the berries in a close-able container (I'm only using a jug and a plastic bag, which is good enough, but a big jar or bottle would be better!). Now leave this container for about 2-3 weeks. After a while you will see that the schnapps is turning red, and that's when it's done!
Step 3. Pour the liquid into a nice bottle/bottles. Keep the berries in the container, they're still filled with juice which you can squeeze out by pouring lots of sugar on them. The chemical reaction will make the liquid in the berries come out. Leave the container a week or so, until the berries are all wrinkly and dry. Then pour the remaining fluids into the bottles.
Now you're done and can serve your deserts with some nice sloe liqueur!