Board Thread:Manga/@comment-38736963-20190329123421/@comment-1327106-20190329204507

If you're looking for a scientific explanation for how the paths work and put people in Titan bodies... there isn't one.

If you're just asking based on what's known in-universe, Subjects of Ymir can be turned into Titans by being injected with Titan spinal fluid; the generic syringe/injection we've seen a few times in the manga. If gassed with Zeke's spinal fluid in particular, they can be transformed via his "scream" (presumably because he's a descendant of the royal family). All generic pure Titans in the series were turned into what they are via one of those two methods.