Start by cleaning everything with a rag dipped in solvent. Then drop in the axle in with one cone on and lay the wheel on a table, supported by the rim so it lays flat. Add to the height of the supports until it's high enough that the axle comes through slightly higher than the bottom of the cups. Add some grease and drop in the balls, moving them into place with a small screwdriver. The axle is plugging the hole so you don't drop an ball through.

Now with the grease keeping the balls in place, lift the wheel, drop the axle in with the cone on the finished side, flip and repeat. When finished, thread the other cone down and you're done, other than final adjustment.