1. Go to the API Console.

2. From the projects list, select a project.

3. Open the console left side menu and select Billing.

4.  Click "Manage Billing Accounts"

5. Next, create account

6. Create a new billing account, and click "Continue" button

7. Add the payment method, by choose credit / debit card and fill in the details.

Once done click "Submit and Enable Billing"