The seeds will range in size depending on the variety of zinnias you are growing, but most will be brownish in color, have a slightly furry texture, and will be somewhat flat and either rectangular in shape or shaped like tiny arrowheads. To identify the seeds, follow the petals down to their base. The seeds will be located in cones just behind the base of the petals.
After removing the heads from the stems, "cure" them on a tray for a few days in a dark, dry area before storing them in a paper bag and sealing them in an airtight container until planting. If you want offspring with the same genetic traits as the parent plants, it's best to grow and save the seed from heirloom varieties. On the other hand, saving seeds from hybrid zinnias will assure you a random mix of genes that may result in flowers with some very interesting characteristics. In either case, strive to save the seeds from only the hardiest flowers with the longest lasting blooms and the most vibrant colors.
Ellen