Voxel-based registration

hi everyone!
I am new at 3D slicer. I try to registrate the CBCTs of mandibular at different time with the modules “CMF registration no growing”, to measure the change of condyle. However, I have no idea to make the “Baseline Segmentation Mask” and the “Followup Segmentation Mask”. Until now, I have made the segmentation of the “stable part” mandibular body.
Looking forward to your answer!