Haworthia seeds can be sown right after harvest. The shorter the wait, the more active the seeds are and therefore the higher the chances of successful germinations. Seeds are best sown within 6 months and when the temperature is moderate.
The best germination medium is pumice of particle size 1-3 mm. This is smaller than the recommended size for mature plants because seedlings have shorter and weaker roots and elongate better in smaller particles. If pumice is not available, perlite can be used as a replacement.
Fill at least ~3 cm of the germination medium in a clean container with drain holes. Sow the seeds on the surface and then cover them with a thin layer (~3 mm) of the germination medium. The top layer stabilizes the young seedlings when they start to put down roots. Soak the container in a solution that has 1/4 strength of the fertilizers for mature plants and 1/2 fungicide of a regular application. The water level of the solution should be ~1 cm below the surface of the germination medium.
How to sow seeds evenly
Mix the seeds with a small amount of the germination medium in a container and shake well. Then lay the mix evenly.
Soak the container in the solution for 1 day. During this period, keep the container in an enclosed space, probably in a bigger container, to keep pests like fungus gnats away from hatching eggs. After that, take the container out and seal it in a clear zipper storage bag. This will keep the seeds in a fully saturated humidity. Keep the container in a bright place without direct sunlight. The first seeds will germinate in ~3 days and the germination goes on during the next ~3 weeks.
The sealed container should be periodically inspected to prevent fungus affection and leakage of moisture. After ~1 year, the seedlings will have ~5 leaves and can be taken out for transplanting and managed as mature plants. Within 1 month of the bag removal, keep the seedlings slightly wet all the time with regular sprays to let them gradually adapt the drier environment.