The foto's show that the knife goes under the dust cap at the inner ring of the ball-bearing. So be cautious and don't destroy the dust cap. Then dry caps and ball-bearing. Then put the dust caps in place. You will feel that they 'click' in place.
Test turn the ball-bearing.
Replace the both for that wheel or downtube, not just one. Use the rubber hammer to put the ball-bearings in again. Do NOT forget the spacer.