You can also poke the balls in from the fixed cup side, through the hole, up into the fresh grease. The grease will hold them while you populate the bearing. If they try to slide out, slip your pinky up into the hole to keep them in place. When complete, put the spindle in and they will stay put while you finish the adjustable cup side.

Take your time adjusting the preload on the bearings. Many times, when you tighten the lock ring, you lose the preload/adjustment. You have to futz with it to get it right.