Unplug the Sonicrafter. Turn the top washer on the auger counterclockwise, also turning the auger screw. Release the top washer when the screw is far enough out to grab and unscrew it with your fingers.
Slip the square indent on one side of the Fein blade adapter onto the end of the Sonicrafter auger.
Slide the desired Fein blade on the other side of the adapter in the desired position.
Put the adapter's top washer onto the top of the blade. Slip the adapter's Allen head screw through the top washer, Fein blade and adapter into the auger. Thread the screw clockwise into the auger, finger tight. Finish tightening the screw with the adapter's Allen wrench.
Plug the Sonicrafter into a power outlet. Turn on the Sonicrafter and make any desired cuts.