Most cactus seeds germinate very easily and quickly, within two weeks. The Opuntioids and seeds in the Pediocactus/Sclerocactus group are more tricky. For growing on, the only "difficulty" is that the seedlings like humidity, plenty of water, and bright shade rather than full sun. I germinate inside plastic bags and keep the seedlings there until they are a good size, anything from a few weeks to many months. I microwave the wet soil before sowing to eliminate fungi and algae.