Determine if the seeds of the amaranth plant are ripe. Take one of the flower heads between your hands and gently shake or rub. If the seeds readily fall from the head, the plant is ready to be harvested.
Wear gardening gloves to avoid contaminating the seeds and to avoid having the seeds get stuck to your hands. Place a bucket or wheelbarrow under the flower head to collect the seeds.
Grasp flower heads between your hands and shake or rub vigorously to cause the seeds to fall into the collection device.
Place the collected seeds in a cool, dry place to settle for 24 hours. Blow dry air with a compressor--or just use your mouth--over the collected seeds to remove loose chaff that came off the flower heads with the seeds.