Shake sugar and water together until the sugar is dissolved. Open the bag and add about a half a teaspoon of yeast. Push as much of the air as possible out of the bag. Seal the bag again. Set the bag aside and let the yeast go to work.
Yeast grows quickly in water that feels a little warm.
If the water is colder, the yeast will grow more slowly. If the water is too hot, it will kill the yeast. The flat shape of the plastic bag has a lot of area in proportion to its volume. It will come to room temperature quickly. If you want the yeast to work more quickly, you may want to wrap it in a towel and put it on top of a heating pad or hot water bottle.