做and 干 both has the meaning "to do", whicle 做 has much broader meaning than 干， depending on the object follows it, for example: 做饭 to cook 做老师 to be a teacher 做蛋糕 to make a cake 做朋友 to become friends 做文章 to write an article

in general and i repeat in general, use 干 when the objective is vague, use 做 if want you wanna express is specific. for instance, 干事业，干什么，干嘛， whereas we say 做蛋糕，做饭，做一番大事业。 hope it helps

as far as i see them,it's a matter of custom.in many conditons ,u can use either of them.but sometimes u'd better use做or干,otherwise,it'll be a little strange.