How do I prepare for insurance application?

Hi All, I'm new here and have just begun my journey. I'm going to a few seminars and have a few docs. picked out. I'm worried about getting insurance approval (I have BC/BS HMO). What 'evidence' do I have to provide? I have been on a number of diets but don't have any 'proof' of them. I've joined gyms but dont have the paperwork for that either. I am worried I can't show that I deserve/need the surgery. Is it enough just to say I've tried diets, tell them my BMI and show that my weight is making me at risk for other health problems? I'm from Long Island, any help/advice/recommendations would be very much appreciated! thanks! Laura ([email protected])