Well, I state how it happened for me, though with the confession that my approach is probably atypical. The first thing I did was to have a complete orchiectomy. So then my body basically had no hormones & it became a medical necessity to prevent osteoporosis / cognitive deficit / etc. Then, I legally changed my name, documents, gender marker, etc. which was easy enough b/c of where I live and b/c I had had the orchi. Then I brought up the need for hormone replacement with my primary care physician due to my body not producing its own hormones and since I was legally female, she prescribed me oral Estradiol & treated the situation the same way she would have had she been treating a postmenopausal woman and/or someone who had had a hysterectomy.
In reflection, there were pros and cons to approaching it in this manner-
Pros- No one batted an eye when prescribing hormones, insurance covered & still covers my HRT, and it *may* be safer to have avoided using anti-androgens & also always utilizing a lower amount of estrogen, the amount a doctor would give to any other woman who needed HRT.
Cons- Making the orchi be the first main step in transition meant that there would be two surgeries which just overall sucks (not to mention the fact that my surgeon did the orchi the way he usually does them for men which in the long run complicated my SRS results), and my physical transition has been prolonged with only subtle changes happening over the years (due to using a much lower amount of estradiol than what some use while transitioning).
As far as therapy - I was in therapy throughout this entire ordeal, I had to be in therapy long before transition, thru transition and probably forever (due to mental health issues non related to GID). My therapist supported me throughout the entire process & at times, even "brainstormed" with me so as to help me know how to navigate my way to & thru surgery. The main role she played was in advocacy - she told the surgeon he could do the orchi, she told the DMV to change my license, etc. etc. So, in my personal experience, my therapist did not directly make anything happen but DID support me.