This capacitor probably won't be the strongest, just saying.
So, here's what you need:
1x empty water bottle
Start by filling the water bottle 70-80% full with the vinegar.
Next, poke a hole in the top. You may use a nail and a hammer.
Put the wire through the hole, so it goes well into the vinegar.
Bend the top of the wire, so it doesn't fall through.
Wrap a good amount of tin foil around the outside of the bottle, and tape it in place.
Connect one alligator clip the some of the exposed tin foil.
Connect the other one to the exposed wire
Connect the ends of the clips to so one goes to each of the leads of a 9v power supply. Take note of which clip goes to positive!
Leave it on for an hour.
Now it's ready! It should look like the pictures above. Good work :)