Here's my favorite recipe:
2 cups lemon juice - you can use fresh lemons or lemon juice concentrate, I usually use about half of each
1 cup sugar
9 cups water
In a small saucepan, combine the sugar with one cup of the water. Stir over medium heat until all the sugar dissolves. Set aside to cool.
When the sugar mixture has cooled, pour it into a pitcher. Stir in the lemon juice and remaining water. Taste for tartness. Add more sugar if you want a little extra sweetness or more lemon juice if it's too sweet. Chill until ready to serve.