How did goku become super saiyan 2

when goku was dead he was training in other world so he can become strong and he did and by that point they tryed to get him to transform into a super sayian 2 and it work and when goku had to fight vegeta goku went back to train for 3 hours or something and then he became super sayian 3 :D then he had to stalk for time intill trunk was heading to the hide out >:D so goku had to fight majin buu and when goku left Majin buu killed his master :O p.s dont mess with majin buu >:)