With old-school graded readers. Turns out that the people in past centuries who we ridiculed for their antiquated approaches for learning languages all spoke multiple languages.
They didn't have comic books, though, which are another good source of interesting reading material that also comes with elaborate visual hints to what is being said. If I were trying to learn Mandarin, I'd scour the internet for bootleg scans of Jademan comics from the 80s.