For a 2 litre I would try:
Big mug of sugar
1/2 teaspoon yeast
1/4 teaspoon bicarb (if you want to - probably only needed with soft water)
The sugar is where the carbon comes from (C6H12O6 --> 6 H2O + 6 CO2), if you only put a few cm in it would run out too quickly. Probably want a fifth to a fourth of the bottle full of sugar. 1/2 teaspoon of yeast should be enough - it breeds quite fast. People say bicarb makes the system last longer - don't know if it's true as I always use it (have never run out of my first tin!).
Use warm tap water, about 25 degrees C. You should get 3 good weeks out of it. If you run two you can stagger them by a week or ten days, so that you're always starting a new one as the other runs out, but then it's twice as much work!
Good luck,
Mark