I learned a lot from a YouTube video from CandiFLA. It was pretty easy. Basically this is how I learned to do it:
1. I spoke in the highest, squeakiest voice I could manage. I concentrated on remembering the exact position my throat was in. In practice this meant my voicebox was raised a fraction of an inch and my throat was "squeezed".
2. I would then position my throat exactly that way, but speak normally. Everything that came out of my mouth sounded female when I did that.