For any intending immigrant coming to the U.S. from another country, the way to apply for and obtain U.S. lawful permanent residence (a "green card") is through the process called consular processing. Its fundamental pattern is as follows:
1. When your spouse, relative or employer submits an immigrant petition (Form I-130 or I-140) on your behalf, they fill out Section 4, Processing Information, to notify USCIS at which consulate you intend to apply for an immigrant visa.
2. Once the immigrant visa is approved, the USCIS sends an approval notice along with the petition to the National Visa Center (NVC), located in Portsmouth, New Hampshire.
3. The NVC uses an internal system to "estimate" when a visa number might be available to you, which is adjusted every month based on the Visa Bulletin. When your priority date become current the NVC mails you a bill first, upon payment of the fees, an instruction packet that also includes necessary forms.
4. The NVC assigns a case number to your application, scan your documents into a database, and places your case in a "waiting list" which is based on your priority date, country of birth, and other information. When your priority date becomes current, you proceed.
5. After the National Visa Center (NVC) determines that supporting documents are completed and a visa is available, then the NVC:
• Schedules Interviews for all immigrant visa issuing U.S. Embassies and Consulates General, with the exception of the Consulate General in Guangzhou, China, which schedules its own interviews, and:
• Sends your documents to the Embassy/Consulate- NVC sends the immigrant visa petition and all related forms and documents (which were submitted to NVC) to the appropriate U.S. Embassy/Consulate abroad, for the immigrant visa interview, and:
• Sends an appointment letter to the applicant, petitioner, and agent/attorney of record (if applicable) to notify them of the date, time, and location of the interview.
Notice: Upon receiving an appointment letter, it is important that you prepare for your immigrant visa interview(s).
6. Once an immigrant visa number becomes available, the NVC requests a visa numbers for you and then forwards your application to the American Embassy or Consulate designated on Form I-140 or I-130.
7. The consulate will examine your application and documents and either revoke the immigrant petition if errors, misstatements or fraud is found, or schedule an interview appointment for you and your family members.
8. Prior to attending the interview, you must undertake a medical examination. The appointment letter will include a list of authorized doctors.
9. The interview is mandatory. You and your dependents must attend. Take to the interview all passports and any original documents required by the consulate. A consular officer may ask questions about your employment history, your prior visits to the US, your H1 or F1 details, your finances (especially income from US source), etc., during the interview.
10. If this is a marriage case it is strongly suggested that the petitioner attends the interview, to establish the bona-fideness of the relationship.
11. Assuming everything is in order, your consular processing case will be approved and you and your family will be issued an immigrant visa. You normally have up to six months from the date of issuance of the visa to enter the United States.
12. When reach the port of entry, you will deliver the unopened "Visa Packet," which you received when your immigrant visa was approved, to the CBP officer.
13. After being admitted, you should receive your green card in the mail within 30 days. If not, you should your attorney or make an InfoPass appointment to check the status of your green card.
• Foreign nationals who are outside the United States
• Foreign nationals who are already in the US but choose consular processing over adjustment of status
NOTE CAREULLY – If you are in the United States illegally or out of status, you should not travel abroad unless you consult with an experienced immigration attorney. Failure to do so may create a 10-year bar to returning to the United States.
If this is you case read more in I-601 Visa Waiver page.