I've always packed them by hand and spent a lot of time making sure to get grease all the way down to the inner race, then butter the outside lightly.
A few years ago, I started a new technique- I put a large wad of fresh bearing grease inside a new Ziploc bag, drop the bearing in, zip the bag, and then squeeze and moosh the grease into the bearing without getting my hands nasty ;-)
Pull the bearing out, and its already buttered, too. Then you can drop the next bearing into the same bag and repeat, adding more grease if you get low in the bag. Works great!