I was diagnosed mostly by my psychologist over a period of a couple of months of formal and informal testing and then I had a 2 hour structured interview with a psychiatrist for confirmation. With my psychologist I did the DES (dissociative experiences scale), ACE score (adverse childhood experiences) and TSI (trauma symptom inventory) among others, I also had full IQ testing. Some of them were checklist type things and others she asked the questions as part of conversations. My psychologist was gathering this information over a period of a month or two.
Once she was fairly sure of her diagnosis she shared it with me. It took me a while to agree with it. Then my T asked if I would see a psychiatrist for more testing to conform the diagnosis.
Once I agreed to that my T gathered historical information from my medical records and previous encounters with mental health and social workers and put it together with her records and then sent it to the psychiatrist.
I had the structured interview with the psych, and the final report including the confirmation of my psychologists diagnosis came back a few weeks later.
It was a hard process for me but luckily it only has to be done once.

ETA: most of this happened in my normal therapy sessions. I needed two longer appointments - one for the IQ testing and one for the structured interview with the psychiatrist. So it wasn't much extra time away from work for me. I guess it depends on how flexible your T can be with session times.