I worried about this for months before coming out - the right time, the right words, etc. In the end, I just blurted it out while drunk. I don't suggest being drunk when you do it, but I do suggest just blurting it. I think it works better. It's a hard thing for parents to swallow no matter how you feed it to them.
Just blurt it out and give them time to digest and answer any questions.
Oh and 'hinting' didn't work. I kept throwing out conversation openers, reminiscing about the boyish childhood, making her watch trans movies, the works. Never produced an opening. I had to just come out with it.
Also - you want to be firm with this. Make it clear this is non-negotiable.
Good luck man!