To choose the best cacti seeds, it will help to know the many different types of cacti that are available, the sizes they can grow up to, and then decide which of these you would like to propagate. One thing you will need to keep in mind about growing a cactus plant from seed is that this can be a rather slow process. The seeds can sometimes take several months to germinate, and it can take years for the cactus to mature; plants from saguaro cactus seeds, for instance, can take several hundred years to reach their full size.
The cactus seeds can be either purchased from a seed merchant or obtained from existing succulents in a cactus garden. It is usual for cacti to either produce seeds in a dry form or encased within a fleshy fruit. These cactus fruits can be planted whole as seeds, but it is best to let them dry out properly in sunlight first. After they have been dried, the cacti seeds can be stored for a long time without losing their viability.
Once you have selected the cacti seeds you want to propagate, you will need to plant them in a flat tray filled with potting soil. A mix of gravel, sand, perlite, and vermiculite is commonly used. The seeds should not be sowed deeply, but with just a light covering of the potting mix. If you are planting several different species of cactus, it will be a good idea to plant them separately and label the planting trays accordingly to keep track of things.
After the seeds have been sown, they should be lightly watered and the planting tray should be covered with a transparent plastic cover to retain moisture; a glass cover or a cellophane sheet will do too. Ideally, the planting tray should be placed in a warm area where it can receive adequate, but not direct sunlight. The cacti seeds should be watered regularly, but without allowing the soil to get too wet and without allowing too much moisture to accrue inside the plastic cover. In some cases, you may need to spray fungicide to keep the seeds healthy.
The cacti seeds can take a while to develop. Once the seedlings have made their appearance, they should be removed from within the miniature greenhouse. Each seedling will then need to be replanted in an individual container. From hereon, they will not require quite as much watering, and they will be able to tolerate direct sunlight.