Remove the rinds of the citrus fruit. This is called "zesting" or "removing the zest".
Bring the zest to a boil then reduce the heat and simmer for 3 hours.
You can drink it hot or let it cool then refridgerate and drink it cold.
Use any type of citrus fruits you have on hand. Keep experimenting until you find the mix you like the most. You can also use "clean" water if you have that. Wikipedia's List of Citrus Fruits
Feel free to add other ingredients or even sweeten it. Organic honey is a good choice.
If you use a lot of zest in a small amount of water it will be potent and can be consumed like cough medicine in small doses, for example, 1 tablespoon each morning. If you used a little zest in a lot of water you can drink it as is.
