Instruct your actor to stay as far away from the background as possible. You need at least 6 feet of separation so that the reflected light from the background doesn't spill over onto the actor, causing him to look desaturated around the edges.
Turn the camera's zebra on. The zebra setting creates stripes over any objects that is clipping, or pure white.
Adjust the camera's aperture until the subject is properly exposed.
Examine the camera's LCD or viewfinder image to make sure the background is completely covered with the zebra stripes. If not, increase the light on the background.
Shoot at an angle that will keep the background behind all parts of the actor, his clothing and any props he's holding at all times.