1) Record yourself on a cell phone for about 5 minutes per day and listen back
2) work on inflection, pitch and rhythm as females speak differently than males
3) Make sure the voice does not come from the chest cavity. It is about pinching midway through your vocal chords and not using falsetto. This requires you train them.
After a while it becomes intuitive and foolproof. I am never misgendered on the phone and am always addressed as Miss or Madame 100% of the time. I also don't need to force to have the voice remain at this range. I can speak this way for hours.
I recorded a short sample here and please forgive the fact that I was a bit out of breath when I did so :)