This is the first part of a 3 part series in which I ramble on about my thoughts on language learning. I’m no expert and I barely know one language well. In a nutshell:

Teach as much as possible in the new language, focusing on vocabulary.

Rather than starting with baby books, which might not be a bad idea, try to use a similar approach but assume the learners know a bit about how the world works.

The goal is to get to about age 5-6 level in vocabulary so the learner can then switch to language books in the new language which already exist.