According to travel.state.gov
After USCIS approves your petition, they will transfer your case to the Department of State's National Visa Center (NVC) for pre-processing. The first step in this processing is the creation of your case in our system. Once this is complete, we will send you a Welcome Letter by e-mail or physical mail. With the information in this letter, you can log in to our Consular Electronic Application Center (CEAC) to check your status, receive messages, and manage your case.
at this time we just have to wait for the welcome letter.