All year round
Fill a pot with a moist, gritty, loam-based compost, or cacti compost. Gently firm down and level the surface. Scatter your mixed cactus seeds over the surface, taking care not to sow them too thickly.
Gently sprinkle a thin layer of vermiculite or fine grit over the seeds, covering the whole surface of the compost. Leave the pot in a greenhouse or on a warm windowsill, covered with a clear plastic bag to preserve soil moisture.
Seedlings will develop within a few weeks. Remove the plastic bag and water when the compost becomes dry, but spray the surface with water regularly, to keep it moist.
Many seed companies sell packets of mixed cacti seed, so you're never quite sure what you're going to grow. If you're looking for fast results, go for a seed mix of fast-growing cacti.
The following spring, your seedlings should be ready to transplant. Use a fork or spoon to gently tease out individual cacti, taking care not to catch the spikes on your skin.
Part-fill a small pot with gritty compost and gently ease the cactus seedling into place. Use tweezers to ensure the seedling is not sitting at an angle.
Fill around the seedling with compost and water well. Add more compost if necessary and then use the spoon to arrange gravel around the seedling. Keep in a sunny spot such as a windowsill, and pot on as and when you need to.