Lemons were cheap enough so I bought a small 2lb bag today and tried out some homemade lemonade. I used the ratio of 1 cup sugar: 1 cup water: 1 cup lemon juice.
Mix 1 cup sugar and 1 cup water in a saucepan and boil gently until sugar is dissolved. Meanwhile, squeeze lemons until you have 1 cup of lemon juice. Combine lemon juice and sugar water. Add 3-4 cups of water.
We got about 1 1/8 cups of lemon juice and just used that with the 1:1 ratio of sugar and water. It was a little tart so the 1:1:1 ratio is probably good. I also read that if you use Meyer lemons they are naturally sweet so you may want less sugar.