Stumbled on this recepie ones when I left a bottle of lemonade standing for a few days. It was raspberry and organic without preservatives or artificial sweeteners. It turned out as a fizzy, slithgly alcoholic and delightful drink.
Note that the end product will contain a slight amount of alcohol.

Step 1: What You'll Need (or What I Used)

5 teaspoons of Sugar - only if the juice is unsweetened
1 packet of dry Yeast
2,5 teaspoons of Dishwasher detergent. The cheap stuff, avoid ultrashine and whatnot.
5 Plastic Soda bottles at 1,5liters
Measuring cups for teaspoons, liters and halfliters.
Ecological Juiceconcentrate, Enough to make 5 liters. Absolutely no artificial sweeteners or preservatives
Water

Step 2: Clean Your Bottles

Clean out the bottles by pouring half a teaspoon of dishwasher detergent into every bottle, filling them with hot water, put the cap on and shake. Put the bottles aside for now.

Step 3: Get the Yeast Going

Put the sugar (5 teaspoons) into a one liter beaker, add the packet of dry yeast and fill up with lukewarm water to get one liter of culture going. Stir it a bit.

Step 4: Rinse Out the Bottles.

Empty the bottles and rinse them twice or thrice. Make sure you get ALL that nasty chemicals out.

Step 5: Lets Fill the Bottles Bit by Bit :)

In a measuring cup measure up the amount of juice for every bottle and pour it in the bottle.
I had half a liter of strawberry and another half of black currant to be mixed 1+5. So I put one deiliter of each in every bottle.

Step 6: Add Culture.

Do the same with the yeast culture. Measure out the amount you need and pour it in the bottles.

Step 7: Add Water.

Now then .. There's enough juice in every bottle for one liter of drink. With the yeast there's 4 dl in every bottle and to get a liter we add 6 dl of lukewarm water.

Step 8: Shake It Up ..

Next you shake the bottle. Not much even turning it upside down once or twice is good enough.

Step 9: Now We Wait ..

Just let it sit with the cap firmly on for about a week. Check up on it every now and then mostly to make sure it doesn't explode. If the bottle seems to bulge a lot release the cap gently (if you unscrew it to quick it could fizz all over the floor)
When it seems clear just put it in the fridge over night or until its as clear as it should be.
Serve chilled and taste before you give it to your friends.
I just set this batch for a party next saturday so I'll be back with pictures of the finished product later.

Step 10: UPDATE!!!

Okay .. so these have been brewing for a few days now and they nearly exploded.
The solution is to:
1.After shaking twist the cap off and just screw it back on gently, not hard. It needs to let off some gas.