A letter can certainly help you get your thoughts in order. That doesn't mean you have to give it to them, you could refer to it if you feel a face to face discussion is a better approach - the letter would make sure you didn't leave anything out.
Don't expect them to understand or to accept what you are telling them. Do expect there is a chance that they will get upset. Usually coming out to close family members is a long, long process. They need to process the information (even if they are supportive) and may have many doubts themselves.
I'd suggest you tell them clearly what you would like to do and how you would do it (what's involved) and how they can help you in that process.
Quote from: IrishPotato on December 09, 2015, 04:09:21 PM
I'm not close with either of my parents but I can't stand having to listen to my birth name and being referred to with she/her pronouns.
Understandable, but don't expect that will change especially if they are not supportive or accepting. They might come around but it is a struggle for many people with their families.