Lavender germinates most evenly if seeds can be collected in the autumn and sown on the surface of a seed tray with bottom heat maintaining 4-10°C (40-50°F). The seedlings are then overwintered in a cool greenhouse or cold frame with good ventilation. Seedlings can then be potted on as needed.
Another method is to start the seeds indoors in winter, planting a few seeds in a few pots with sterilized seed starting mix. Dampen the mix, press the seeds into the surface, insert the pots into plastic bags, and put them in the freezer for about a week. Let them come to room temperature on their own, and then use bottom heat as indicated above.