Adjusting the Mesh...
Often the custom mesh will not be scaled or positioned the same as the original survivor's mesh. Adjusting the custom mesh to match as closely as possible to the original survivor mesh will help make the rigging much easier to do.
The general method I use:
Scale the whole custom mesh from the bottom between the feet (usually the origin of the mesh) so that the knees match with the original survivor mesh
Scale from knees up to match the pelvis bone area
Scale from the pelvis bone area up to match the shoulders
Scale the neck and head to match the eyes
Pose the custom mesh's armature to match original mesh's pose
Apply the auto-weighting
Tweak rigging via individual vertices or vertex groups