I know this isn't a popular opinion, but my approach has always been to be completely transparent as early as I can, but still to do it gently.
In my very short experience dating as a trans woman, I've noticed there are so many straight guys who are into trans women, but the majority don't like feeling 'deceived.' As far as I'm concerned, I'm no different than a cis woman, but a lot of straight guys don't see things this way at first, and end up feeling lied to (as unfair and horrible an experience as that might be for us on the receiving end). When they feel this way, they may pull away even if they had originally thought they could date a trans woman.
When your cards are on the table right away (but again, laid out in a gentle and genuine way) they are less likely to feel weird about it. At least that's how it has worked with me.
In terms of practicalities, you may try something like...
"I really want us to get to know each other better, but there's something I'd like to be fully transparent about mostly because I do really like you/care about you, and I respect you... I was born a boy, even though I never felt like one. In my heart and soul, I've always been a girl, and once I was able to, I made physical changes to live the way I've always felt inside. I understand if you haven't really come across this in the past, but please know this doesn't change any part of what we've had so far. I still like you very much, and hope you feel the same. "
Or some variant thereof

. Of course, all of this is up to you, but you asked for a specific "how to," so I thought I'd let you know how I've phrased it in the past. It's not perfect, and it's not truly reflective of what it means to be trans, but it's a start. You can refine your explanation of things once you start getting to know each other