Use a pair of rubber gloves. Spray WD-40 on one glove. Using that glove, grab out a glob of grease. Use the other hand to roll and mash the grease into the bearing until it seems like it's squishing out all over. Throw a bit of that glob that's left over into the bearing race to hold it in place. Press the bearing into the grease, and finish putting everything back into place.
Oh, and if you have to remove the bearing races, DON'T simply hammer the new ones in place. Put a piece of wood or something over the thing and try to hit it in the center to evenly seat the new races.