With the actor in a set position, keyframe the channel data at frame 1. Your character should be scaled to roughly fit your actor's proportions, and be in the same pose. The receiver cubes should have the same position relative to the character as the receiver on the actor. Take the Null under the hip receiver and constrain it in position and orientation to the top of the character hierarchy. Add a second null to the receiver on the torso.
Position constrain the first null to the midpoint of the spine and the other to the effector.
Relax and invert. Constrain the Null in Position and Orientation to the head articulation. Relax and invert. Same as head. Two Nulls, one on the elbow articulation, one at the wrist.