In a small, dark-colored container, combine the vitamin C powder and the water.
If you are only using water and vitamin C, you are done. This will store for up to 2 weeks or longer in the fridge.
If you are using glycerin, dissolve the vitamin C in 1 teaspoon of water, then stir in 2 tablespoons of glycerin and store. This version will last a month or longer.
Personally, I like the pure vitamin C and water recipe as a toner after cleansing. However, glycerin is moisturizing and softening for the skin and will extend the recipe. DO NOT use glycerin on anything that is used in oral care or in the mouth as it is bad for teeth.