A small magnet helps to get ball bearings out for cleaning (although it has been argued that magnetizing in this way is not good for the steel balls since it could cause them to cling to eachother rather than roll freely in the hub). I use the magnet, going with convenience over concern about magnetization.
I use a thin pickup magnet to remove the old bearing balls but install the new ones using large tweezers. I'm not worried about magnetizing the balls but the tweezers let me position the new ones more easily. I apply a heavy bead of grease to each hub race before adding the balls as it "glues" them in place until I can install the cones and locknuts.
To the OP: the only "trick" to rebuilding cup and cone hubs is the final bearing adjustment. You want to leave a TINY bit of play which will disappear when the wheel is clamped in the dropouts. The best way to learn this is to have someone familiar with the process go over it with you the first time.