In general I'd suggest either:
I suggest these things because, while developing a feminnine voice does come easily to some people, for many other people it's challenging, complicated and subtle. And, as you point out, it's very possible to over practice and hurt your vocal cords or to reinforce bad habits.
EVA MtF is great for the $$ and is made by a trans-specialized speech therapist. It's not as good as seeing someone face-to-face but is SO much better than going by random videos or advice from the internet! It may seem expensive, but compared to buying even a single lesson with a speech therapist is very cheap. If you don't have an iOS device, consider buying a cheap used one!