Unplug the Dremel rotary tool.
Press the collet lock button, near the tip of the Dremel rotary tool, to stop the shaft from spinning. Remove the collet nut from the tip using the Dremel wrench, which was supplied with the Dremel rotary tool. Remove any attachments and the collet from the shaft.
Unscrew the housing cap near the tip and remove.
Hold down the collet lock and thread the square "Driver" nut into the hole where the collet and collet nut were. Lock down using the supplied Dremel wrench.
Maneuver the flexible drive shaft of the Flex-Shaft 225 into the square hole of the driver nut. You may need to turn the end slightly to allow unobstructed entrance. Lock the Flex-Shaft in place by screwing the large, plastic retainer nut onto the threads that previously held the housing cap.
Attach the collet, collet nut and attachment on the far end of the Flex-Shaft 225, just as you would the end of the Dremel rotary tool. There is another collet lock button at the end of the Flex-Shaft to keep the shaft from moving when tightening the collet nut.