A guru is someone who has acquired the skill to be a guide, a teacher, or a master of a specific knowledge field. Gurus are more than teachers or guides because they have so much knowledge that they're the SI unit of knowledge in the field they've specialized it. A guru's voice is effortlessly knowledgeable because they've mastered their skill. Hence, a guru is a person people refer to when they're talking about those who are most knowledgeable in given fields. As you can see, a guru is an acquired title that doesn't come easy.
A guru knows so much that learning from them is like drinking from the well of knowledge. Moreover, the beauty of learning from a guru is that they've mastered the art of teaching. Have you ever read such a technical book, but somehow you understood the message. Technically, that is how it should feel about learning from a guru. A guru voice should give you the urge to want to learn more.
A guru's voice is ideal for self-help projects. It has a straightforward tone that is not condescending. It should inform you that to be better, you have to do the work. Give your project a specific title, so people know what to expect. Self-help is so diverse; thus, you cannot lump several topics into one project.
A guru's voice is characterized by undeniable insight. If you have read self-help books before, I bet you've experienced light bulb moments on concepts you thought you understood but didn't. It should also be profound because that is the impact immense knowledge should have on people.