Look at my approach of it. I started top clean you general layout for mobile, putting some 20px default margins globally and removing your negative margins on inner elements. hen removing the fixed sizes of the form container, then playing a bit with the fields.
You should get the general idea of the process. Remember that rules you define for mobile don’t apply for upper devices. Inheritance of properties only go down, not up. So you’re safe to style for mobile as you like.