This is how I did it:-
500g sugar, 250ml water, 250ml lemon juice
Combine sugar and water in a saucepan. Bring to the boil, stirring, over gentle heat. Boil gently for 5 minures without stirring. Cool, then add lemon juice. Strain and store in a covered container in the fridge.
This makes a concentrate to dilute with water when ready to drink.
It`s lovely with freshly picked lemons.
I tend to squeeze the lemons and then add the same amount of water (which is the same as 237SJ has said......but works with any amount of lemon juice). Then I weigh out (in grammes) the combined volume of juice/water in ml.