making fun of fat girls is sad.. just means your dick is likely to tiny to hit it doggy style.
You don't.
Just like with any girl, don't be a douche. Talk to her and listen to her and be honest with her. Actually care about what she says. It never fails. You don't always have to go blackhat with girls! Whitehat is better in the long run and won't leave you in a sea of regret.
chloroform works wonders