When I was about to come out to my wife, this is what I did:
First, I read about it, a lot. I read about crossdressing, being transgender in general and watched a whole lot of YouTube channels, which was really more for helping me understand it better before I told my wife. If you want to check out some good YouTube channels, I suggest checking out Vera Wylde (married crossdresser. she makes a lot of tutorial type of videos but also occasionally talks about her relationship with her wife, though I don't think she is as active as she used to be on YouTube, still, her videos are pretty helpful) and Aurora Andrews and PrincessJoules (both are transgender, I know PrincessJoules is post-op and Aurora is either still pre-op or recently had her SRS, I've neglected watching YouTube lately so I'm not 100% where she is in the process, either way, they are both amazingly awesome and I love their channels, Aurora is actually quickly becoming one of my role models and PrincessJoules became a role model for me almost instantly, which is kind of funny since they are both a little bit younger than me, lol).
When I was close to being ready to tell her, I tested the waters. First I shaved my private area which I knew she would enjoy and she did. Next, after a couple of weeks, I shaved my legs. She liked this as well, but was a little more surprised. Then after a couple more weeks, we were sitting in our living room and I started talking to her about it. I've always had a few feminine traits (the way I carry myself, which wasn't overly feminine but also wasn't distinctly masculine either, I also have always had a tendency to sit in "feminine" ways, such as tucking my feet under my body and resting my hand on the side of my neck as apposed to the masculine way of balling up my fist and leaning the top of my head against my fist). I asked her if she had noticed that I had a "bit of a feminine side" and when she said yes I informed her that it wasn't so much that I had a "bit" of a feminine side, it was that I had a full blown feminine side. I explained to her that giving my up bringing and conservative parents and the conservative town I grew up in, I was never really able to fully explore it and it was something I always had to repress. Then I told her a little bit about my crossdressing tendencies and that I wanted to start exploring it more and I wanted to be able to do so around her. Hind sight, I should have made it clear from the beginning that I wanted her to be a part of it as well, but I didn't think of this until later. I wanted her to be a part of it, I just didn't think to mention it right away. After she got more used to coming home to be wearing a dress, makeup and a wig, I started opening up about how I didn't think simply dressing up would be enough for very much longer. I never lead her to believe that transitioning was NOT a possibility, but I never explicitly said that it was something I was interested in. Once I told her that I want to transition, she accepted it and we have been working on it for almost a year now. I am on track to start transitioning sometime in the next few months, but I have been taking it slow so that she can cope with it and handle it much easier. She has been AMAZINGLY supportive.
But the bottom line is communication. Keep an open dialog, explain to her that this is something you feel like you need to do and let her know that it does NOT change the way you feel about her. Let her know that you still love her and that you want her to be a part of it.
Also, be ready to show her any material you find about the subject. I was lucky enough that my wife was at least somewhat familiar with it, mainly because when we first started dating we would go to a local bar (which unfortunately no longer exists) and watch the drag show on a semi regular basis, so the crossdressing aspect wasn't very unfamiliar for her, the transgender part was and I always made sure she knew she could talk to me and I always have made sure that I never kept anything from her in regards to it. As I said, and as I'm sure everyone can tell you, communication is an absolute must. You can write a letter as some have suggested, and if you do you can let her read it or you can read it to her if that helps. I never thought of going the letter route (though I am strongly considering it for when we tell my parents) but the writing a letter and reading it to her or letting her read it herself will help you organize your thoughts before hand. I would suggest, though, that you keep the letter to the main points. Don't go into too much detail, just use it to open the door to the conversation. Allow her to ask questions, answer them honestly and let her know she can always ask you questions or voice any concerns she may have. I've read a lot of horror stories about people telling their significant others, but I've also read a lot of really positive stories. More often than not, the process can bring a couple closer together.
Hope this helps and good lucky honey. Be strong, be ready for rejection while hoping for acceptance and just do what you need to do to be happy in life. Your life is YOUR life, no one else's and no one will blame you for looking out for your best interests and trying to maximize your chances of being happy in life and happy with who you are.