Once the custom mesh is lined up with the original survivor's mesh and skeleton, you can rig by transferring weights from the original survivor mesh to your custom mesh. Some tweaking of the weights is usually still needed after the transfer.
Rigging via automatic weights provides a quick way to get most of the weighting done. Some tweaking of the weights is usually still needed after the initial automatic weighting.
The mesh is all blue and there are no vertex groups listed in the right panel, indicating no weights have been assigned to the mesh.
Select "Object Mode".
Select the custom mesh.
Hold down Shift key and select armature of the original survivor.
Press Ctrl P to bring up "Set Parent To" options.
Select "With Automatic Weights" (under "Armature Deform").
Select "Weight Paint" mode.
Select the "Bip01_Head1" vertex group (the vertices of the head).
The non-blue colors show various levels of weighting for the vertices. blue is 0; green is higher, then yellow, then orange, and finally, red is 1. Usually, the head should be all red, so we'll change that next.
Select "Edit Mode".
Select the "Bip01_Head1" vertex group.
Deselect any currently selected vertices.
Click on the "Select" button under the Name box under the list of vertex groups.
Adjust the "Weight" bar under the "Select" button to show, "Weight: 1.000".
Click on the "Assign" button to the left of the "Select" button.
The head is now all red, as it should be.
Verify the other vertex groups (one per bone) the same way we just did for the head and tweak as needed.
You only need to rig or weight bones with names that start with "Bip01_". It's okay if the other bones have weights, but you don't have to worry about them, because the survivor animations won't animate them directly.