While easy to propagate through seed, lavender is most often grown via cuttings. Lavender seed has a short shelf life and not all gardening centers carry it. To further exacerbate the problem, many of the most popular varieties are hybrids that produce sterile seed or no seed at all. But once you get over these hurdles, you will find lavender seed fairly easy to sow although slow to germinate.
Place the lavender seed you intend to use (take into account lavender seed's frequently low germination rates) into a Ziploc bag filled with an inch or so of moistened seed-starting soil mix. Leave the bag in the refrigerator for three weeks or, if you are in a hurry, for one week in the freezer.
Fill the planting tray with high-quality potting soil, then press it down gently with your fingers to firm it.
Moisten the soil by submerging the bottom of the tray in water until the top of the soil is moist. Then allow the soil to drain for one hour or so.
Sow the lavender seed by sprinkling a few seeds in the middle of each cell. Then cover the seed with a thin layer of soil--no more than 1/8 inch--because lavender seed needs light to germinate.
Place the seedling tray in an indoor, sunlit spot that remains at around room temperature. Keep the soil moist by re-wetting it following the method in Step 3 whenever it dries out. The seed should germinate within two weeks. If they do not, place them in an area where temperatures are around 50 degrees F for two weeks. Then move them back to room temperature to stimulate germination.
Transplant the healthiest seedlings into their own small pots (2 inches by 2 inches or so) when they have reached roughly 3 inches tall.
Transplant your lavender seedlings out of doors in three more months (after a two week hardening-off period) when the weather is warm.