If you are outside the US, you can get a green card through Consular Processing. If you are currently in the US you can adjust your status to permanent residency. If you are outside the US, only option is consular processing at a US Consulate overseas for an immigrant visa. Once your petition is approved and you get the visa, you can come to the US and your green card will be mailed to you in the same way had you adjusted your status in the US.
While applying through consular processing, the USCIS will forward your approved petition to the National Visa Center (NVC) for further processing. The NVC, in turn will then send you a package containing forms, supporting document list and other instructions. Once done with the application process and when visa numbers are current for your preference classification for your country of birth, a notice regarding your appointment including more forms and instructions will be sent to you. You are required to attend a personal interview for you and your dependent family members accompanying you to the US will also have to attend one at the consular post. Appointments are scheduled on short notice, so it is advised to be ready and be able to travel when the appointment is scheduled. Also remember that you have to take a medical examination with a physician and the physician should be approved by the US consular post.
The supporting documentation needed for Consular processing is more or less similar to that required for adjustment of status. However, there are some exceptions. For consular processing, it is mandatory that you provide a police certificate of good conduct from each country in which you, or your family member resided for six months or more since the age of 16. You must also submit proof for having fulfilled any obligatory military service.
The consular processing is comparatively quicker than the adjustment of status process and can take as little as six months to complete. But remember that since the process takes place outside of the US, if there is any problem with the application, you may not be able to return until the problem is settled. Not to forget that there is no formal appeal process for consular processing applications as there is for Adjustment of Status applications. Maintain your US non immigrant status while the application for an immigrant visa is pending. Note that you or your family members are not eligible for EAD cards or Advance Parole.