You screw off the collar on the tool, remove the chuck and inner part. The chuck is replaced with a special chuck (no inner piece). The special chuck has a square opening in it's nose that accepts the square end of the flex shaft inner cable. Is is actually a speedometer drive cable, from yesteryear.
Then you pull the inner cable out slightly, if it will, and put the inner cable into the square opening of the special chuck on your dremel motor. once engauged, you carefully screw the outer shaft housing on to the end of your dremel motor.
What you want to avoid is damaging this area during flex shaft assembly. I have had success with turning the output side of the flex shaft, while tightening the outer flex shaft onto the dremel motor. Stop tightening if it starts binding...
Rotation seems to help with alignment of all parts.

Cheers,

Doc