Make the mixture (btw
, I always used 2 cups of sugar). Then let it set for 24 hours. After 24 hours, if everything is sealed correctly, gently
squeeze the bottle. It should feel like an unopened bottle of soda. When you squeeze a little more, bubbles should come out of the air tube in the tank. If neither happens, you have a leak.
What I did for the mixture was put in two cups of hot water, added 2 cups sugar, shook up the bottle until the sugar dissolved (I use an undrilled cap for this). Then I added 2 cups of cold water, added 2 tsp. yeast and 2 tsp. baking soda, and shook up the bottle again. Then I added the final 2 cups of cold water, put the cover on, and then set it up under the tank. Bubbles would be coming out within 24 hours.
If you find that your setup is leaking, you may want to try to make a new cap for your bottle. I drilled a hole with a bit exactly the same size as the tubing (I believe 5/8"?) and forced a piece of rigid tubing into the cap, with about an inch sticking out on either end. I ran a bead of silicone on both sides, smoothed the silicone with my finger, and set aside for 24 hours. It worked perfectly.
One site I read suggested putting a pin hole in the 2 L bottle, but DO NOT DO THIS! That tiny hole was the source of much frustration. Once I got a new bottle without a hole, it worked fine.