Growing cacti from seeds is a job for the very patient gardener. It can take up to 15 years to grow a large cactus to maturity from seed. If you want to multiply your cactus collection, try taking cuttings--a far faster way to get new little cacti.
The best time to take cuttings is in spring when the plant is most prone to grow. Use a sharp, non-serrated knife to cut pieces from the cactus you want to propagate. It's easiest to take cuttings from columnar or segmented cacti. Make a sharp clean cut; don't saw.
Cuttings won't grow upside down. If you are taking several cuttings from one cactus, flatten the top cuts and angle the bottom cuts so you can tell them apart.
To avoid fungal rot, allow the fresh-cut areas of your cuttings to dry out before planting in soil. Allow anywhere from a day or two to a week or more for this process. Or dip the cutting into garden sulfur before sticking it into the medium.
Use a well-drained potting soil that contains equal parts of potting mix, pumice and clean sand. Give the potted-up cuttings full sun and cool temperatures. Water in the same way you would water a fully established cactus--scarcely. In a few months your cuttings will take root.