This recipe is simple.
All you have to do is save the peels of any citrus fruit you eat. That’s because the peels of oranges, lemons, and other citrus fruits have 3 times more vitamin C than the part you usually eat.
Take those peels and remove the bitter white pith. The easiest way to remove the pith is to scrape it out with a grapefruit spoon. (You only need to remove the pith on the thicker peels like on navel oranges. If you’re using thinner peels, like those on tangerines, you don’t have to worry about the pith.)
Once you’ve removed the pith, cut the peels into small pieces, and set them out to dry for a few days.
You’ll know the peels are finished drying when they easily crack and snap in half, rather than bend.
https://www.nutrientinsider.com/News/3891/How-to-make-your-own-vitamin-C.htm