https://www.jeffreymorgenthaler.com/how-to-make-your-own-tonic-water/
Ingredients
4 cups water
1 cup chopped lemongrass (roughly one large stalk)
¼ cup powdered cinchona bark
zest and juice of 1 orange
zest and juice of 1 lemon
zest and juice of 1 lime
1 tsp whole allspice berries
¼ cup citric acid
¼ tsp Kosher salt
Combine ingredients in a medium saucepan and bring to a boil over high heat.
Once mixture starts to boil, reduce heat to low, cover and simmer for 20 minutes.
Remove from heat and strain out solids using a strainer or chinois.
You'll need to fine-strain the mixture, as it still contains quite a bit of the cinchona bark.
You can use a coffee filter and wait for an hour or more, or do as I do and run the whole mixture through a French coffee press.
Once you're satisfied with the clarity of your mix, heat it back up on the stovetop or microwave, and then add ¾ cup of agave syrup to each cup of your hot mix.
Stir until combined, and store in the attractive bottle of your choice.
You now have a syrup that you can carbonate with seltzer water; I use my iSi soda siphon for some nicely-textured bubbles.
To assemble a gin and tonic, use ¾ ounce of syrup, 1½ ounces of gin and 2 ounces of soda water over ice.