Begin by going to your insurance website, search "criteria for panniculectomy" & "criteria for abdominoplasty" and print it for the doc.
Then, if you have pain, rashes etc....caused by the skin, go to pain management or another doc, NOT a PS, get a referral from that doc to PS for surgery. Bring the insurance criteria.
It can also be helpful to write your own letter to go with the PA request. Mention co-morbidities that are resolved, how exercise is crucial to maintenance and keeping co-morbidities gone, how the skin affects those things.
Best of luck!