I use a hand primer and ever so gently seat primers on already loaded ammo. Be very careful. If its pistol caliber, I like to take the barrel out of the gun and drop the loaded rounds in to test them. don't get discouraged, it gets easier. What calibers are you doing?
Your kidding right?. . . . .please don't tell me you seat primers on cases that have been charged and bullets seated?. . . . . .why in Gods name would you do that?