The BNC does not get terminated directly onto the CAT6 cable. This is what baluns are for. They normally have screw terminals that receive the CAT6 (or other UTP cable to that matter) conductors. On the output end they have the BNC bayonet. You then connect a camera with a BNC-BNC cable, and repeat the whole setup on the opposite end of the CAT6 cable.
For more info on the hook-ups, please see the installation manual the the actual balun model you will be buying.
Once again, BNC does not get directly terminated onto the category cable, so you will have to use a balun unless your cameras have baluns onboard (in which case you will see the screw terminals on the back of the camera).
Dmitri Abaimov, RCDD