America's Test Kitchen has an extensive YouTube presence. I was surprised and delighted to find a cocktails section. They had a piece on making your own tonic - so I figured that I would try. The idea is from then, the ratios are my own. Citric acid is often used in canning, so you can find it near the canning jars. Cinchona bark is a specialty item that we ordered from Amazon. It comes in the right sized pieces.
Ingredients
2 oz Cinchona bark
2 Lemon grass stalks, chopped finely
2 Thai lime leaves
Zest of 1 Seville orange
Zest and juice of 2 lemons
32 oz water
small pinch of salt
2 cups of plain white sugar
4 T Citric acid
Method
Combine bark, citrus, Thai lime, salt ina 4 quart saucier. Bring to the boil. Turn heat down, simmer for 40 minutes. Leave to cool for 24 hours. Strain (through cheesecloth and a fine mesh strainer) and add the sugar, and citric acid. Dissolve the sugar and citric acid. Allow to cool. Transfer to a refrigerator safe bowl, cover and chill overnight.
This will keep for several weeks as long as you keep it well covered.