Put the return track cover back on.
Put a light film of grease inside the nut.
Stick all the balls into the nut in the correct places.
Make a roll of shim just under the core diameter of the nut and balls and then slide the nut onto it making sure the balls stay in place.
Offer this up to the screw and wind the nut back onto the screw shaft making sure the nut is the right way round.
If necessary, wash the grease out with paraffin and apply the correct lube.
Sit back, drink beer.