When applying for a nonimmigrant visa at the Embassy in London, you must complete the DS-160 application form before booking an appointment to attend the Embassy.
When you begin completing the DS-160 application form online, make a note of the Application I.D. number first, as you will need the number to book an appointment, and also if you need to retrieve the form at any point.
This is the important bit! You must submit the application form at least 24 hours before you attend the interview. If you do not submit the form and receive the confirmation page (see below for example) at least 24 hours before you attend the Embassy, the Nonimmigrant Visa Unit may be unable to process your application and you will have to book a new appointment to apply on another day.
Remember, once you have submitted the form, you cannot make changes to it. You should carefully review the DS-160 form to ensure that all the information you have provided is correct before you click “Submit”. If, once you have submitted the form, you have to change any of your answers; you will be required to submit an entirely new form. Remember, if you do submit a new form; make sure you do it at least 24 hours before the scheduled interview.
If you submit a new form after you have booked your appointment, you will not be required to book a new appointment, just bring the confirmation page of the new, correct DS-160 form and advise the consular officer at the time of your interview.