Thanks for the tag,
@JustGeorge. Of course, I'm late to the party as usual.
Much has already been said in previous posts, and I concur with
@Vinayaka's statement that jnana is wisdom gained through direct experience rather than from a book. The only thing I'll add to that is a book can offer insight into jnana yoga (the path of knowledge), but it's the actual experience that manifests the wisdom. As I said on the forum previously, scripture merely validated my own experiences.
Think of it in terms of skydiving. One can read every book and magazine about skydiving, but one won't actually know what skydiving really is if one hasn't jumped from a plane.
This is a great question, and again, I concur with what
@Vinayaka said. For me, it was a spontaneous mystical experience that ultimately set me on the path. But for those who have yet to have such an experience, I would say bhakti and karma yogas are the best start, and ultimate will lead one to jnana. I would go as far as to say that bhakti yoga and karma yoga are actually a part of jnana yoga in the sense that the experiences brought about by bhakti and karma can ultimately lead to jnana.