People who you anticipate will bring problems might actually be supportive, and others whom you expect to be understanding may be difficult. I have come across middle managers with supposedly set sexist attitudes who have turned out to be very supportive aids when approached with professionally presented facts. Later when the workforce had undertaken awareness raising workshops about transsexualism she was quick to realise that I had given good advice, the support raised was much wider than anticipated but from sources other than she expected. I know some people who have successfully informed their own inner circle of colleagues several months prior to transition at work, but would point out that these were genuinely close friends whose attitudes and philosophical outlooks were well known and understood. Remember that the tone in which information is given affects the way in which it is received, and try to be matter of fact without being off hand.
Respect breeds respect.
You ensure that salient points are covered . Having things written down clearly can also be a real godsend when the moment of disclosure arrives because of the human tendency in most of us to dry up, or run round in verbal circles, when nervous. This is your opportunity to encourage the adoption of your preferred words and create shared terms of reference. You may also have a preference for Gender Dysphoria or G.I.D. with regard to clinical term. However anxious you feel inside if you are seen to be taking a reasoned and moderate approach you are more likely to be treated in a reasoned and moderate way.