Inspect the bearing races on the spindle and in the cups. If shiny and smooth with no pitting, you can re-use these parts. Replace the ball bearings. Make sure the spindle and cups are really clean before greasing and installing the bearings. Most BBs take 1/4" balls. Get the 'Grade 25' bearing balls. You can either pop the old bearing balls out of the cages and replace them with the new, or just stick the loose balls in the cups with grease. Removing the cage will allow maybe two more balls on each side - typically you remove a 9-ball cage and replace it with 11 balls.