This is the rule people argue with most, so it is worth being precise about what it means. It does not mean writing does not matter. It means writing is a second skill, and running it first will teach you to say words the way nobody says them.
What goes wrong
You already have a set of rules for turning letters into sounds. They are your first language’s rules, they are automatic, and they are wrong for the new one. Read beaucoup before you have heard it enough times and you will store something built out of English vowels — and then spend months unlearning the version you invented.
The written form feels like the safe, solid version of a word. It is not. It is a second representation of the thing you actually want, and it arrives with interference attached.
What to do instead
Hear it first, and hear it more times than feels necessary. Say it back. Let the written form arrive afterwards, once the sound is already sitting there — at that point spelling becomes what it should be, a label for something you know, rather than instructions for guessing at it.
This is why entries carry a pronunciation field and why the audio button sits next to every word. Not decoration. The point of the row.