BrainTreePayments create new payment method

I am going through the BrainTreePayments documentation page and I came across the Payment Methods. The documentation says "To create a new payment method for an existing customer, the only required attributes are the customer ID and payment method nonce."

The question is

1) How do we create a new payment method for a new customer without payment method nonce? 2) How do we verify the validity of an existing payment method?

via Shawn Ang