I don't remember how I first brought it up... largely I just answer "yes" to all the "forms of abuse experienced" on the intake, but wait to develop trusty before I can actually talk about it. With the t before this most recent one, I was seeing her at a sexual assault clinic, so it was a given we'd talk about it. It was hard even with her though. One day I ended up writing down details and asking if I could have an extra session because I needed to tell her something heavy. I went into the extra session and read it to her... it wasn't everything, but it started the heavier stuff.
I'm going to start again with another new t in the new year (grumble, grumble. Students leaving. Grr). I'm not sure how it will come up, though she will have access to my chart from previous t, so she will be able to see there's stuff to talk about.

Good luck however you decide to broach the subject. Sending strength your way.