It is necessary to remove three items from the nose of the rotary tool: the housing cap, the collet nut, and the collet nut.  First unthread the housing cap, then hold down the collet lock button and unthread the collet.  Once removed from the tool, you will also see the collet.  Remove the collet.  Along with the Flex Shaft in the package you will find a six sided nut with a square hole (this is called a driver cap).  It’s easier to see the square hole if you hold the nut up to the light and look through it.  Again, hold the collet lock down and carefully thread the driver cap onto the bare metal threads of the high speed rotary tool where the collet and collet nut were removed.  To make the Flex Shaft attachment functional, the collet should go into the end of the hand piece on the flexible shaft and the collet nut should go over the metal threading on the hand piece end of the flexible shaft.  To tighten the collet nut on the hand piece, press the silver shaft lock button on the hand piece of the flexible shaft.  This will stop the threaded area from turning as you tighten the accessory you want to use in the hand piece.