Disclaimer - I have created this based on my own experiences of coming out to my partner, and also the experiences I have gathered from other people and places on the internet. It is not a guaranteed how-to guide and should be tailored to suit your partner. I bear no responsibility for anything that happens if you decide to follow these tips, and also do not guarantee that your partner will accept you. Each person is individual and while my partner accepted me, yours may not. Please bear in mind that every person is different and what ive written below may not work entirely for you. If you do decide to use some of what ive said, put it into your own words!
The below are only suggestions on how to approach this conversation with your partner based on my experiences. This is not a how-to guide!
First of All
To start with I thought I would give you the biggest bit of advice I can give you that applies throughout this whole experience. Take things at your partners pace. Do not force information or changes on her at all otherwise she will feel overloaded and may freak out. When coming out to her answer her questions but do not add information, if she goes silent then dont try to talk to her as she is thinking things through. Let her sit there and wait for her to speak. Give her choices wherever possible, try to make her feel she is in control of the conversation as she will feel like her life is being turned upside down.
Another thing to bear in mind is your approach. Dressing up and surprising her when she gets home from work is probably the worst way you can do this. At best have a photo handy of you dressed up, dont force it on her but have it in case she asks and feels the need to see it. I had one on my phone which I didnt even show her in the end. Also dont have a photo of you all made up looking glamorous and amazing, if she has any insecurities this will likely make things seriously worse. Make it an average pic without makeup or something, it needs to be enough to make it real without making her feel bad about herself.
There is never a good time to bring this up, try to make sure she will have plenty of time to absorb and process what you are telling her.
I recommend you read something like 'Something to Confess' by Karen Adler, you can find it on Amazon in Kindle format or Smashwords for other formats and its only £2. It is written by the wife of a Crossdresser and describes her thoughts and feelings throughout the experience of him coming out to her. Be warned, she comes across as a very open minded person so your partner may not be as understanding. However it was still an emotional and painful experience for Karen therefore it is still a useful read so you can understand how your partner will feel at various points and how your actions will probably affect her. There are other books you can read to but this is one I have personally used.
Take the time to have a look on Trans support sites and forums, and possibly get involved too as there are people on there who will have been through the same and will be able to offer support and their experiences when they came out to their partners. They will also be able to offer advice on your approach and maybe even support you through the emotional fallout during and after. Be warned though that telling your partner you have done this could bring up trust issues. She may understand you needed to do this for support, but may feel like you trust strangers more then her.
Preperation is Key
Be prepared for the worst. As well as you may know your partner all bets are off when it comes to revealing something like this. Be prepared for the possibility that she may go and tell all your friends, family and workplace about this side of you. By coming out to her, the rewards could be beyond your wildest dreams, but at the same time it could also be a life changing experience where you will lose this relationship and you will truly find out who your friends are.
Be prepared for questions. The number one thought that will go through her mind is 'are you gay?', and I was asked this quite a few times when I came out to my gf. Now we all know that just because we dress doesnt mean we fancy blokes too, gender and sexuality are seperate things. However to the general public the only contact they have with people who are Trans are drag queens who are essentially gay men who dress to entertain, or camped up actors on comedy shows mocking Crossdressers (Little Britain comes to mind). These are the things your partner will have in mind when you tell her this about you. Being Trans is very misunderstood, and I even told my gf in some ways it would probably have been easier to come out as gay because at least everyone understands it.
Be prepared to be honest (well, almost). Well now the cat is out of the bag you might as well be as honest to her as you can. Tell her why you dress, what you get up to, and how it makes you feel. If you were like me and are suffering with depression because of hiding it, tell her. If you are having gender identity issues and are not sure how far you want to take it, go see a therapist (before telling her if you can) and tell your gf you are too. Its important she knows how it has been affecting you, be as honest as you can but also very importantly do not force information on her. You do not want to overload her. Answer her questions at her pace. There may however be things you might want to keep back depending on what you do when it comes to your dressing. For example, if you dress in her clothes do not tell her that as she will feel like you have violated her personal space as well as deceived her. Ideally do not dress in her clothes at all, your partner (whether you tell them or hide it) will make the connection
between your body type and theirs and think back to how strange their
clothes fit (women notice when their clothes feel different) and then it
will be a violation on a different level. Some women are very insecure
in their own bodies and will see it as the ultimate betrayal to have a
man in their clothes or wearing femme gear as it will feel like their
man is stealing their sexual identity and will feel as though they have
nothing of their own. It makes it worse for some when they see their
"men" looking better than them.
Be prepared to reassure. It cannot be understated how important this
step is, my gf has mentioned several times that the only way she got
through this was because I reassured her continuously. You need to tell
her you love her, and how much she means to you. Tell her she is the
most important thing in your life and the reason you are telling her is
because you want to be with her forever. Tell her that
knowing how important dressing is to you, you could have taken the easy
way out and split up with her. But instead you took a risk which could
have cost you your friends and family, but were prepared to take it because
you couldnt imagine your life without her. Assure her you are the same person you always have been, that you still have the same interests, loves and hates, that this is just something extra on top of all of that. Its always been a part of the way that you think, and as such has always been a part of the man she fell in love with.
Be prepared to offer support. Have support and information available for her if she needs it but do not force it on her. Give her choice to try and give her a sense of control. And in a very chaotic and disrupting moment in her life, she will need some sense of control to save her sanity. She will most likely find it extremely difficult to comprehend and process what you are telling her. One place I can suggest is Transpartners, it has been setup by the partners of men who are Trans and offers possible explanations, links to other resources, and also a forum too so partners can get in touch with people who are also in the same situation as them. I would recommend reading Transpartners yourself first so you can be prepared for any questions she may have from reading it. There is also a partner specific section on The Angels forum too. If you are lucky and have made friends who are out to their partners on other Trans sites, they may offer their phone numbers to support both of you too. While I offered these to my gf, she did not want to speak to strangers about our situation so I was not able to find out how effective any of them are, nor can I recommend them. My gf chose to speak to her best friend about it, and while I was not too happy with it it turns out this was the best move because her friend sorted my gfs head out and has also offered her support to me too.
And finally be prepared to be patient. Dont expect her to be accepting in a day, she might need to take some time to figure out what to do. You will have just turned her world upside down and she probably wont get her head around it in a day. Dont force the issue but also make sure she doesnt bury her head in the sand. You will have to judge this yourself.
After the Conversation
If you have made it this far and she still wants to make it work, then this is excellent news but you are not in the clear yet. There are still some things you need to bear in mind. First off, just because she accepts it does not mean she is happy or comfortable with it. When she is ready you will need to discuss boundaries. Tell her what you want to do when it comes to your Crossdressing (clothes, shaving, going out, etc) and find out her thoughts on them. Be prepared to compromise with her, do not expect to get your way entirely. It may be she wants to go on shopping trips and have girls nights in, or it may be that she doesnt want to see it at all but at least gives you some freedom to dress when she is not at home. At least you will not be hiding it anymore and believe me that is a massive relief.