Measure the height of your California king headboard. Divide the height by two. This is your halfway point.
Mark the measurement of your halfway point onto the wall by the bed with a piece of chalk by drawing a horizontal line all the way across. Run a studfinder across the wall and mark where the studs are by drawing tick marks on top of your horizontal line.
Drill in the bottom part of a flush mount along your horizontal line where your studs are. Ideally there should be studs every 12 inches or so and thus you should use around six of the bottom parts of your flush mounts.
Turn over your California King headboard. Drill in the top part of your flush mounts along the middle of the headboard, installing them 12 inches apart or however far apart your bottom mounts are.
Pick up your headboard and slide it on to the wall, listening for the top and bottom flush mounts to interlock and hold the headboard steady.