Surprised nobody has answered this simple question. Just take non flowering shoots around 3 or 4 inches long and insert them into a compost with half sand or vermiculite. Keep in a cold frame.
Another way is to bury the lavender plant in gritty compost so that the tips of the stems are just protruding from the soil. After a couple of weeks or a month, they should start to root into the soil and can be removed and potted on.