Lithops are short day plants belonging to the Mesembs (Mesembryanthemum) group. They require temperatures below 20 °C to germinate. I germinate them in autumn and sometimes in early spring. Protect the young seedlings from intense heat and sunlight, especially when you sow in spring.
I have excellent germination results with 100% pumice (Bims). Put a layer of 1-2 cm pumice in the germination tray, moisten it thoroughly, close the lid and sterilize in the microwave for 1 minute at 900-1000 Watt. Let the temperature decrease and then open the lid and sprinkle the seeds on the pumice. Do not cover the seeds! Close the lid and position your tray in the windowsill or other cool bright spot to germinate. Seeds should have been sprouted within 2 weeks. This is the time to remove the lid. Make sure the pumice with the seedlings stays moist untill the lithops plants are a bit older.