I have collected a fair number of elder flower cordial recipes. Some take tartaric acid, others citric acid but they all take fresh lemon juice and varying amounts of sugar. With practice, I am sure, you will soon be creating your own.
In a suitable plastic container dissolve the citric acid in the boiling water and then add the rest of the ingredients.
Leave covered at room temperature for at least one day. Strain and bottle or freeze in suitable portions. To serve dilute about one part in seven with water, add some slices of lemon and ice cubes.