There are many kinds of Amaranth. The information below refers to Amaranthus hypochondriacus, known as Love Lies Bleeding or Red Cockscomb. It is the one preferred to medicinal usage, and for the seeds.
Collect leaves when the plant is coming into flower. They are then dried for infusions and extracts. The seeds are harvested when ripe. The best way to collect the seeds is to tie a small bag over the seedhead (I like to use pantyhose because it lets in the light and lets out the water). The seeds will simply fall into the bag when ripe enough, so collection is easy. Note: if you don't collect the seeds, you will soon have zillions of new plants - it self-seeds readily over vast areas, and can quickly become a weed. It also attracts grasshoppers in their swarms. You have been warned!!
Cook leaves like spinach. Seeds can be popped like corn by placing 1 tablespoon seed in a dry pan and stirring till popped. It can be used as a cereal by combining 1 cup seed with 3 cups cold water or stock in a pan and bringing to the boil. Simmer for 25 minutes, covered. It has a very bland taste although some say it is nutty. The flour can be used in cookies, crackers and baking mixes to provide protein. The toasted seeds are added to soups, salads and stews.
Seeds are used to treat diarrhoea and gastroenteritis. Once used to stop menstruation and for contraception, now used to treat excessive menstrual flow. Externally used to treat swellings, sprains and tick bites. A tea made from the dried flowers can be used to treat excessive menstrual bleeding. A tea from the leaves can be used to relieve stomach upsets.
Warning: Avoid use if pregnant or lactating.