Put your sugar in your pop bottle. Fill the pop bottle with the warmest temp the yeast will tolerate (110 F). Shake to dissolve the sugar completely. Pour a small amount of that mixture into a cup (preferably pre-warmed but if it's thin walled plastic like a measuring cup I would skip this step). Sprinkle your yeast into this liquid and give it a gentle stir. WAIT UNTIL THE YEAST IS FOAMING. 10-15 minutes usually. Then add this mixture back into your main bottle. Add your pinch of baking soda now if you want to. Seal and go.
This. Treat the yeast like you would if you're baking bread. It really helps to proof it first because it starts bubbling faster and you can double check that your yeast is alive.