Thank you to Rosita Arvigo for sharing how she grows and collects amaranth seeds, and how she uses the flowers and seeds in cooking.
I have personally collected amaranth seeds many times over the 35 years of living in Central America. My variety is A. retroflexus, a common food crop here.
The stems can be cut for greens several times as they sprout right back giving twice as many as before it was cut. Mine give leaves for 3-4 months. Then, I let them gradually go to seed - you get many more seeds that way, too - and cut off the seeding tops. By the way, the flowers are delicious to eat steamed or sauteed as well.
I lay the seed heads on a cloth and dry them in the sun for two days, bringing them in every night to avoid dampness. Then, I shake the seeds vigorously in a paper bag and then strain through a sieve or use the wind to remove the outer shell.
Frankly, with this variety, it is an all-day job to harvest about a pint of lovely, shiny seeds.
I roast them and add them to oatmeal or tortillas just a little at a time as they are kind of gelatinous.
Hope this helps everyone. It's well worth the effort, I say.