The capsules, as you've probably noticed, ripen intermittently during the long flowering period, so you can gather seeds several times. You can sow at various times. Sow some seeds at once, and store others for up to a year in a cool, dry place. You can start some seeds indoors in containers in winter and transplant outdoors after frost. Or, when the soil has warmed, just sow seeds outdoors where you want plants to grow.
Germination depends on the seed coat.
Some people nick the rounded top of a seed before sowing to encourage germination. Sow about one-fourth inch deep in a soil that drains well. Texas star, Hibiscus coccineus, likes sun but appreciates some afternoon shade. A sandy, slightly acidic soil enriched with organic matter is ideal. This shrubby perennial likes water during the heat, and responds to a balanced fertilizer.