There's really no easy way to do it. I was 80% sure my boss would be fine with my transition - he's very supportive of LGBT rights, but I worried that it may cause us to lose clients so I wasn't 100% sure. I had already started T when I told him. He asked me if I could get a passport and go up to one of our other offices in Canada for training. I told him I could, but that I'd prefer to wait, as I was planning to change my name in the near future. That made him ask what I was changing my name to, and from there, we segued into my transition.
You could try to do something similar if there are things like name badges or ID cards that you get at work. Ask if you can put a preferred name on there, or if they are difficult to get changed. When he asks why, let him know that you're trans and planning to change your name in the next few months.