After you have completed Step 1 (Choose an agent) and Step 2 (Pay fees), you will submit your visa application form, and collect and submit the required documents to the the National Visa Center (NVC). This process will be explained in Steps 3 through 6.
After the necessary forms and documents have been collected, and when your priority date is current (if applicable), the NVC will schedule your interview at the appropriate U.S. Embassy or Consulate. The NVC will then transfer your case file to the U.S. Embassy or Consulate.