Pick individual outer leaves or wait to harvest full heads.
For ongoing harvest, you can begin harvesting once the leaves reach about 4 inches tall. Harvest the outer leaves of the head and let the inner leaves continue to grow and mature. Just snip either single outer leaves or grab a bunch of them and cut them with scissors one inch above the base of the plant. Be careful; if you cut into or below the base, the plant may die, so leave the small leaves in the middle to regrow easily.
Do not cut more than a third of the plant in you want it to keep growing