1) Take the Terminator arm and trim away the part highlighted in red, aiming to retain the curve of the upper arm where the cable/box meets the upper arm armour itself.
2) Trim away the area highlighted in green, again aiming to retain the curve of the vambrace.
3) Carefully cut along the dotted lines to remove the stormbolter.
5) Holding your blade flat against the back of the Terminator's hand, carefully cut into the area marked A, to separate the hand from the jutting out part of the vambrace.
5) Turn the arm over and place it flat on your cutting surface. Carefully cut downwards across the wrist (along the dotted line) until you reach the jutting-out part of the vambrace. The hand should drop away, leaving the vambrace intact.
6) Turn the arm back over, and cut away the shoulder joint along the dotted line. The line appears curved here, but if you angle the blade correctly, it'll be a single straight cut.
7) This now gives you three separate parts – the hand, them arm, and the shoulder. Dry fit these on your marine's torso to try out different poses.
8) Done. Have a cup of tea. :)