Wait for the grape hyacinth to finish blooming in the late spring. Once the flowers fade, they will form seedpods.
Examine the seedpods when they are brown and dry. If they look like they are ready to split open, you can harvest the seeds.
Crack open the seed pod with your hands and pour the seeds from the seed pod into a letter-sized envelope.
Seal the envelope and write what kinds of seeds it contains on the outside of the envelope.
Store the envelope in a cool, dry place, such as a basement or a garage, until you are ready to sow the seeds.