Put the conical lower burr in place making sure it sets on the pegs of the impeller properly. Then place the washers on and tighten the 10mm nut. Remember the threads are reversed.

IMPORTANT: Tighten the nut pretty hard. The impeller, being plastic, has a small amount of springiness to it and must be squashed hard so that the height of the lower burr does not vary. If you find that the burrs are too close after you're all done, then this is the reason. Tighten until the motor begins to move, and then give it an extra wack.