The process depends on what type of warhead you wish to make. The Hiroshima-class bombs were gun fired fission while the Soviet Tsar Bomba type were fission induced hydro-deuterium helium thermonuclear fusion devices. A fusion bomb requires a conventional atom bomb to reach the sufficent temperature required for hydrogen to fuse into deuterium and then into helium (2-100 million degrees).This warhead then releases many times more energy than the starter bomb alone. The highest energy bomb released so far was Tsar Bomba at 50 Mt. However, if the casing hadn’t been modified then the nuclear yield could have been in excess of 100 Mt.