I'm sorry, but I'm not sure what works best for a beginner. I started my PhD when the original Transformers paper [1] had just been released. I had no background in NLP whatsoever and used the original paper to write a Transformer and the full training pipeline from scratch during the first couple months of my PhD without referencing any existing code (only reading the paper and it's references).

So I'd say, if you're motivated you could do the same. That said, I've always been a self-starter and I started my PhD after working for a decade. I'm sure there are other resources out there, but I'm not equipped to say what's best for a beginner (I found the original paper to be excellent, but most everyone during my PhD, including my advisor, found it to be inscrutable; I think it's written more like an engineering focused paper, which might be why researchers found it difficult to grok, but with my previous industry experience it seemed quite clear).

[1]: https://arxiv.org/abs/1706.03762