Sudden and Gradual in Brief

In short, the basis of ignorance is conceptual attachment, that is, believing in essentially existing things and beings. From that come emotional attachments, and from that all sorts of deluded actions. One can go gradually by first restricting one's actions with the precepts, calming emotional states with meditation and eliminating false ideas through insight. Or one can go directly to the root cause, that is clinging to thoughts, and see that there is nothing to grasp. The result is the same.