4. Plant the seeds in high-drainage soil. Whether you bought seeds or harvested them from an existent cactus, you'll want to plant them in clean, shallow containers filled with suitable soil. Moisten the soil thoroughly before planting but do not allow any standing water to remain. Next, spread the seeds across the top of the soil (don't bury them). Finally, lightly cover the seeds with a very thin layer of soil or sand. Cactus seeds only have a small amount of stored energy and if planted too deeply will not reach the surface before they run out.