Since there is no possible way to adequately explain crossdressing and it's infinity shades to anybody, I always suggest showing instead of telling, and doing so a little at a time. For instance... I assume you shave your legs. Let her see that you do, and just tell her you like the way it looks/feels. Polish your nails, and open up that line of communication. I don't think you should push any boundaries at your new job with feminine clothes or longer hair, but you can certainly feminize your casual off-work clothes. I'm not talking dresses and skirts, but girl's jeans, maybe leggings and a longer top. Just wear things that fit your style without overwhelming her. Start wearing panties, or a least those men's briefs that are essentially panties. She'll see that it's not the end of the world, and you'll ease into conversation about it.
