Once a green card holder's naturalization application is approved, they will attend an Oath of Allegiance ceremony. It’s very important that they complete this step. An applicant is not a U.S. citizen until they have taken the Oath of Allegiance.
After their citizenship interview, they will receive a notice in the mail with the date, time, and location of the ceremony (usually a local courthouse or government field office). The time it takes to schedule the ceremony varies by state.
The applicant will be asked to return their green card when they check in. Once the ceremony is over, they will receive a Certificate of Naturalization and begin their life as a U.S. citizen!
Here is additional information about the Oath of Allegiance.