I tried that, once, with my wife. I wrote a rather long letter that I thought "covered all the bases" and upon reading it she'd understand my situation, have compassion, and be fully accepting.  Not.  It was a disaster.  Too much information, too quickly, without warning.  Don't forget, you've been living with this gender uncertainty and later, knowledge, your whole life.  Cisgender people might consider their gender at times but not nearly as seriously.  Honestly, I don't think most of them get it. 

My suggestion is to tell one or two, separately, verbally, and in private.  Face-to-face is best so you can each see each other's body language and modulate the discussion accordingly. The good news is that you've done your research and know what you want to say - in your letter.  So you're prepared. 

But don't expect instant acceptance and "good for you" all around.  Your friends and family love you and want what's best for you.  They may think that trying to steer you away from being transgender is the right thing to do.  So, be patient, gentle, and stay calm.  

Hugs,

Emma

I hope it goes smoothly and wish you joy and understanding in this.