If you are repacking your bearings, take the bearings and place them on a clean surface like a piece of clean paper. Squeeze a liberal amount of general purpose automotive grease into the center of the bearings. Fill the entire center higher than the top of the bearings. Now take your thumb and press the grease into the bearings.
If you are replacing your bearings, you will pack them with grease in the same manner. Installation is the reverse of removal: replace the bearings, then reinstall the bearing nut, the safety cap, the cotter pin, and the dust cap. Some people like to add a little more grease to the party at these stages. It definitely won't hurt, you really can't use too much grease!