Your therapist should be able to help prepare you, and I'd recommend taking advantage of that.
I used a fairly medical approach when I came out. First step is finding the right person - it could be your boss or it could be a human resources person. Then plan what you're going to say. I kept it pretty simple. Said I'd been going to a lot of appointments lately because I'd been diagnosed with Gender Identity Disorder and that I'd already started taking testosterone and that they'd be noticing some changes in my voice and appearance.
They wanted a timeline and, of course, you can't really give them one about the changes on T, but you can say I'd like to begin working as male around <date> if that works for you. And tell them about any surgery dates you may have. I legally changed my name months before I began using my new name at work. That way I could do it all at once, change my ID and be known as Dennis everywhere.
Then of course there's the washroom issue. We had a gender neutral one so I said I'd use that while the transition was occurring. They may want you to stop assisting in strip searches as soon as you come out to them. Having done a few of those as a customs officer and hated it, that's probably a blessing
Dennis
