I just did one. It was my virgin experience so I was careful and slow
I have a makeshift headset cup press. Basically its a long threaded shaft with two big washers and some nuts. The washers were the perfect size for the Fat BB bearings, and I just worked them in slowly. Remember, if its not going in exactly flush, it may not go in at all 'cause they fit so tight.
Mind the sexual overtones here.