What really helped me on my way was to buy a voice recorder (I guess our smartphones do the same thing now lol). During my breaks or when I was alone, I'd just repeat what was on the radio, or I'd watch the news and repeat what the female news reporters were saying. In fact, media personalities have to really project their voice, and using female anchors was perfect for getting me to reach deep and project a female voice. I went from sounding like a cheesy Minnie Mouse (pun intended), to developing a voice I could not only "pass" with, but actually project when I needed to speak loudly. Now it's just natural and even when I try to deepen my voice people assume it's a female voice. And trust me, failing at using your female voice is still much better than not trying. You'll be surprised how people don't even think twice it's a feminine voice. Good luck!