Look online for seeds. If you can't find seeds, buy a dragon fruit, which can be found at many Asian markets. They come in yellow, red and magenta colors.
Cut the fruit in half. Scoop out a teaspoon of pulp, making sure you get some of the black seeds. Mash the pulp in a small amount of water to separate the seeds.
Clean the gelatinous pulp from the seeds. You won't need many seeds, because the germination rate is high. Enjoy the rest of the dragon fruit as a snack.
Fill a shallow container with a planting medium made from equal parts seed-starting mix and sand. Moisten the mixture. Sprinkle the seeds on the top of the medium and barely cover them with sand. Seeds need light to germinate.
Place the container in a plastic bag to retain moisture. Set it in a warm spot that gets plenty of light. Check daily; seeds start germinating anywhere from one to 14 days after planting. After germination, remove the plastic bag.
Do not overwater the young cacti. After several months of growth, pot each small plant in its own container.