If you have access to organic or at least spray free lemons and oranges keep the skin cut it in small stripes and dry it .Best in a dehydrator (about $100 Sunbeam) or really low temperature in the oven. when they are bone dry use a coffee/spice  grinder and turn into a small grained powder. We did the same with our cherry guavas, removed the inner pulp with the big seeds and dried the rest. Super yummy. Have a heaped teaspoon daily for your vitamin C suppy either straight or over your cereal ,yoghurt or smoothie.