The acidity of the lemon juice slows the yeast activity for this soda, so start it about a week before you plan to serve it.
1. Bring 1 cup water to a boil in small saucepan. Remove from heat. Add sugar and salt, and stir to dissolve both. Cool, then stir in lemon juice.
2. Pour juice mixture into clean 2-liter soda bottle using funnel. Top off bottle with water, leaving at least 1 inch headspace. Taste, and add more sugar if desired. (Extra sugar will dissolve on its own.)
3. Add yeast to juice mixture. Screw on cap, and shake bottle to dissolve and distribute yeast. Let bottle sit at room temperature away from sunlight 5 to 7 days, or until lemonade is carbonated. Check bottle periodically; lemonade is ready when bottle feels rock-solid with very little give.
4. Refrigerate overnight, or up to two weeks. Open bottle very slowly over sink to release pressure gradually and avoid bubble-ups. Store in refrigerator.