First make sure your seeds are viable using the flotation test. Then sow stevia seeds indoors or 8-10 weeks before the last frost in spring. Sow seeds ¼ inches deep in seed-starting formula. Sow double or triple the amount of seed you usually would for other plants, as the germination rate can be lower than for other seed varieties. Seedlings emerge in 14–21 days :)
What important point is the size of seeds are very small and we need to take care accordingly. You can watch the following video with the link: https://www.youtube.com/watch?v=Ll_d0FsBzHM
Further, you can read the article: https://www.everstevia.com/Study-on-Seed-Germination-in-Stevia-rebaudiana.pdf