There's more to all this than just a pair of stockings, a good push-up bra and high heels in your size, for both you and your partner. With that, let's get personal. Whether cross-dressing is primarily a sexual kink or you're finding yourself moving toward a physical transition, this will impact your sexuality and your sex life with your partner. An honest discussion of the nitty-gritty is something that doesn't happen often enough. I'm working on an assumption that you want to maintain your relationship and the two of you are committed to one another. While I'm using female pronouns for the partner here, I think that most of this will apply regardless of gender.
If it's really all about the sex for you, at least for now, working dressing up into bedroom play may not be so hard. Talk it through and define both of your boundaries, just as you would for any other sexual fantasy. This is a good time to talk about what each of you needs and wants from this experience and about your fears and discomfort. Take baby steps and start small. Treat this as role play, making it something fun. Written erotica, a visit to an adult toy store or an appropriately themed video may help to provide the two of you with a bit of inspiration, if you're floundering.
Women's magazines are full of advice about getting yourself in the mood. Take advantage of these tips when the night comes. While it may be a bit sappy, it works if you're feeling nervous, regardless of chromosomes. Send the kids to Grandma's house, relax and have a glass of wine. Laugh together. While no one mentions this, put your lingerie on early in the night, before you have a chance to chicken out! There's nothing worse than breaking the mood when you excuse yourself to put on a garter belt and stockings. We've covered the easy part of this question. Integrating dressing up into your sex life isn't necessarily difficult, especially when it's about sex for you, not gender. Managing life in the bedroom when you're feeling confused or struggling with your own gender identity is hard on both of you. Even if you're not taking physical steps toward transition, you may find your libido is low or you're just not feeling quite right about sex or about certain sex acts. You might even find that acceptance increases your libido. If you are transitioning, hormone treatments and androgen-blockers will change your sexual function in significant ways.
You have a few key responsibilities here. First, you owe it to your partner to communicate openly about what you're feeling about your body and sex, and if necessary, about changes in your feelings. Some T-girls are fine with maintaining a traditional sexual relationship with a female partner, while others may be more comfortable looking for sexual activities that feel more feminine and less traditionally masculine. If you're not okay with using the appendage-that-shall-not-be-named or having it touched, say so. You might be okay with some sex acts and not others or with pleasing her even if you're not comfortable without your panties. Keep in mind that her libido may be just fine, even when yours isn't. Making an effort to find sexual activities that work for both of you can help to preserve your relationship. Women manage to have perfectly satisfying sex with other women all the time, but it can take a bit of extra creativity to get there, if it's new to the two of you. Regardless of whether you're okay with sex or not right now, you do need to prioritize maintaining physical closeness. If sex is a no-go, cuddle, hold hands, give each other back rubs.
All of this implies that there's still a mutual attraction, desire and willingness to work through the challenges that can come with crossdressing or gender identity issues. That leaves several big what-ifs. You may find that you're attracted to men or other T-girls. She may not be attracted to you when you're presenting as a woman. In either case, a trans-friendly counselor can help the two of you stay together or split up as good friends, even if you're no longer functioning well as partners.