The "right" way to learn English (or any language for that matter) is finding a method that suits your learning style. If you like reading, then textbooks and literature are good materials; if you prefer watching movies or listening to music, then those methods will help make the learning process faster and much more enjoyable. Good luck! :)

The truth is, you don't. You have to trust whatever textbook/course you're doing, or trust the people helping you. You'll know later, when you understand the language better, if that learning process was helpful to you or not. For now, just do it.