On my experienced last year for the first time of US passport I made an appt online thru calling there hotline but it was hard to get the answer of your call only voice prompt and even the online appt was really hard to book it.. My situation was I have my itinerary with me and it was an emergency so 14 days before your flight you make an appt online because they don’t allow the applicant without appointment confirmation that they are going to send it to
your email. After I got my appt confirmation I printed out with my itinerary and went to Dallas Passport Agency with your original Naturalization Cert and make copies of it. Then 3 days after I need to come back to pick up my New US Passport and my Naturalization Cert. with no problem. I didn’t mail it because i applied in person and pickup when. It’s ready. But like above said it depends which City you are.. Good luck and congratulations!