I wrote a post about how I used FloFlo for my first book, but the tl;dr is that I did a mixture. I pre-learnt some high-frequency words (drilling down from most frequent) and then added more words as I went, depending on how useful I deemed them.
It’s much easier to learn words once you’ve see them in context, and you can be confident about the parsing, but if you don’t have a big vocabulary it’s pretty helpful to learn a chunk of the most frequent words you’ll see before you start reading.