The Four Stages of Enlightenment
The Buddha taught that Stream Entry is attained when the first three of the ten fetters are overcome (the belief in an independent self -sakkaya ditthi-, doubt, and belief that chanting, rites and rituals alone could lead to Nirvana).
A once-returner is one who has cut the first three of the ten fetters (thus attaining stream entry) and loosened the fetters of sense-desire (greed, lust, aversion), and ill will. Notice that these two powerful fetters need only be loosened to graduate from stream entry (sotapanna) to once-returner (sakadagamin) status!
Our Beginning Zen practices are important.
Our daily practice of metta/loving kindness reduces our ill will. Our daily practice of Present Moment Awareness, and Silent Present Moment Awareness reduces our sense desire and our daily Mindfulness of the Body, the Feelings, the Mind and Mind Objects eliminates our sense desire.
The path that leads from ignorance to Stream Entry is the Eightfold Path and the path that leads from Stream Entry to the Once-Returner and to the Non-Returner is the Noble Eightfold Path according to Venerable Ajahn Brahm, because now it is a noble one who is following it.
And full enlightenment requires dropping the desire to experience the world of form, attained through the jhanas (fetter number six), and the world of formlessness, attained through the immaterial attainments (fetter number seven). And then conceit (fetter number eight), restlessness (fetter number nine), and ignorance (fetter number ten) must still be overcome.