I actually brought up the fact that I wanted to wear women’s clothing to my wife before I actually purchased any. My wife isn’t even 5’ and doesn’t weight 100lbs, so “borrowing” her things was out of the question, and I didn’t want to have female clothing around the house that was obviously not hers and have her find them. She is actually pretty excited over the fact we are both gender fluid. Now I have a quite a few pretty things and plenty of things to wear, many of which she bought me.
I am not sure how adventurous you are in certain aspects of your life, but maybe a little bedroom role play / role reversal could break the ice on the subject as well and make the transition and later conversations easier.
On a slightly different subject, I would suggest if you are underdressing at work, especially if you work for a larger company or in an office setting, make sure to keep in your mind your first step if you get caught, read, clocked, whatever term you want to use, and feel like you are being harassed or it’s being held against you, go to H.R. and just let them know how you identify. There are protections in place in most States / Countries and no large company wants to deal with the internet / social media backlash of discriminating again someone’s gender or gender expression. It’s a hot button issue and your H.R. department is well aware on top of a PR Nightmare, it could very well lead to a harassment suit that you’ll win.