I use 2 cups of sugar and 1 teaspoon of yeast (Fleischmanns) for my 20G tanks.
First I put the sugar in the pop bottle, fill the bottle half way with hot tap water and shake intil sugar is desolved. Then in a separate container I put the yeast, letting the yeast and sugar set over nite so that the hot water cools & the yeast warms up to room temp. Then I add cold tap water to the yeast and shake to mix, then pour the yeast into the pop bottle and turn the pop bottle over once to mix it all together. Then I add more cold tap water to the bottle, only to the top of the bottle label. I then let the bottles set over night and they are ready to hook up. This mix takes about a hour to start and last up to two weeks.