Breast Reduction and Hidradenitis suppurativa - how to get started, what to expect?

I been diagnosed with HS and lately I been struggling with it under my breast area, upper stomach. My DR. And I have talked and agree that breast reduction can benefit me a lot. I honestly don't know my exact bra size tho to DDD sometimes can be small for me. I guess my question will be how to get started, what to expect on the process of getting approved by insurance and what is that like.