After the visa petition is approved and a visa becomes available, the family member called the beneficiary files an application to adjust their status to lawful permanent resident if inside the US or begins consular processing if they are abroad.
If the beneficiary is in the US and is eligible to apply for a green card here, they must file an I-485 application with United States Citizenship and Immigration Services (USCIS), and depending on the relationship, you and your family member must later attend an interview with a USCIS officer.
If the beneficiary is outside the US, or is otherwise not eligible to adjust their status in the US, they must go through consular processing. They will need to file forms with the National Visa Center and attend an interview with a consular officer.
It is always important to consult with an immigration attorney or Board of Immigration Appeals Accredited Representative before filing the I-485 application or before leaving the country for consular processing. This will help to ensure that your family member is in fact eligible to apply for lawful permanent residence. Someone who files the I-485 application or leaves the US for consular processing will be at risk for deportation or may not be able to reenter the US if they were not actually eligible to apply.