I sort of just did it, similar to me deciding to go fulltime before I even began HRT. I had choosen my name, I wanted to go fulltime so I did, that included my voice, at first it was rubbish but I practiced, but I never practiced alone, I did it when I was talking to my friends, making phonecalls, buying clothes, food etc. Trail by fire, but I got my voice to a point were it pass quickly, unless it's someone who known before, or got some kind of knowledge in the area and can tell the difference.