In autumn, cut the lavender cautiously: Just take away the faded blossoms before winter.
At about the end of March you can carry out a contour cut, i.e you shorten the plant evenly.
If your lavender plants are older they might not like a strong cutback in spring. You better fertilise older plants before their growth period start, and cut back the older parts of the lavender when fresh shoots start to grow. Younger plants can be cut down to a third or a quarter. They will then sprout nicely.