How can I activate my multifidus muscle?

Exercise it. There are specific exercises to activate the multifidus muscle. Lie on your back with your knees bent and contract one side of your pelvis trying to move it to your spin, hold for 10 sec and then the other side. Other exercise while on all 4s and lift the right knee and left hand off the floor and balance there for 10 sec and switch. There are great videos on the web. Type in multifidus exercise.
Stand up. These postural muscles are important for stabilizing the facet joints. They are activated as soon as you stand up and assume an erect posture.