Well, I've come out in a few situations.
The most recent two were easy - both were small groups where we literally sat around a table when we met up. I just told them "Just so you all know, I'm going through a transition from female to male, so please refer to me using male pronouns." That was early in and no one really had many issues. It worked well, too. It felt awkward to me personally, but I think that was just my own "ugh, I wouldn't have to do this if I were cis, so tiresome." Lol.
With the job I had a few years ago...uh, that was weird. I basically told one of the supervisors that I wanted to talk to Louis (the boss) about it, and blurted out "I'm having a sex change." I would NOT recommend that lol, because she told Louis on her own. He was very supportive...he called up and said "I just want you to know that this is a free country and you are allowed to do whatever makes you happy. If anyone gives you any problems, let me know and I will take care of it." Then when it came to the coworkers I came in contact with, I just brought it up at some point in conversation. I never had an issue with any of them, again, they were all polite. Actually, one of them that I thought might be rude, it turned out that he was a gay man. And he knew several gay men who dated trans men, so he really understood that we are in fact, men.
It's hard to say if I was lucky or not. I think the best advice I can give is, try not to make a bigger deal out of it than you have to. Bring it up like "hey, just so you guys know..." I find that the less big of a deal you make of it, the less other people care, and life usually just goes on.