A General Method
To germinate cactus seeds, use a fine mix of 1/3 washed sand, 1/3 cactus compost, and 1/3 perlite. Mix it thoroughly and put it in 10cm plastic pots or seed pans (with adequate drainage holes). Gently pat down the mix to eliminate large air pockets and smooth the surface with a straight edge.
Carefully sprinkle the seeds evenly over the surface. Try to ensure you leave a space of at least 2cm between each seed to prevent overcrowding later on. If the seeds are large, simply lay them on top of the soil. If they are very fine, mix them with sand or talcum powder to scatter them more evenly. Cover the surface with a layer of sand, perlite or vermiculite about twice the thickness of the seeds, and stand the pots in a shallow pan of room temperature water. When the water begins to rise to the surface, remove the container from the water. It is vital to maintain a high level of humidity. This can be achieved by placing the pots in a propagator, putting sheets of glass over the pots, or simply putting the pots in plastic bags. You will need to mist the pots daily with fine spray pump bottle. Do not let the compost fully dry out, and do not saturate it either, both extremes will prevent germination.
If possible, you should provide bottom heat, or keep the pots in a place where the temperature can be kept around 25°C, or 33°C daytime maximum and 15°C night-time minimum.
Under these conditions the seeds may germinate in one week, otherwise they may take significantly longer. Cacti seedlings are prone to ‘damping-off’ fungus, a fuzzy mould which grows on the surface of the soil and the plants. An application of Cheshunt Compound whilst misting will usually prevent/cure this. Once the seeds begin germination, stop misting the seedlings and provide water from underneath (as in the first watering).
Once the seeds have germinated, ensure the pots receive adequate lighting, either natural or fluorescent. If you are growing under natural light do not let harsh direct light reach the seedlings as they will burn. A cotton cloth or tissue paper will provide decent shading. If possible aim for a day lenght of 16-18 hours.
Over a period of months, gradually acclimatize the seedlings to warmer, drier, and brighter conditions. Water less often, but apply a greater quantity of water each time. After about six months, the seedlings can be set in the same conditions as other cacti. Also, you can begin applying fertilizer at 1/4 recommended strength.
When they are one to two years old or crowded enough to be just about touching each other, transplant into individual pots. Some quick growing, or very spiny species may need transplanting much sooner.