You will probably find the process of applying for U.S. citizenship (naturalization) to be far less complicated than getting your green card was. It involves submitting only one government form and very few accompanying materials, and attending an interview at an office of U.S. Citizenship and Immigration Services (USCIS) near you. At the interview, you will be tested on your knowledge of English as well as U.S. history and government.
As with any application to USCIS, however, you must take great care to make sure you are eligible, fill out the form correctly, and prepare fully for the interview and exam. For eligibility rules, see Who Can Apply for U.S. Citizenship. Any piece of missing or untrue information on your application or during your interview for citizenship could easily lead to a denial—even if you didn't realize you made a mistake or weren't trying to lie, and even if the mistake isn't about anything you think is important.
Recognize also that there are risks to submitting an application for naturalization. If information emerges that casts doubt on your eligibility for the green card, you could not only be denied citizenship but lose your U.S. lawful permanent residence.
IMPORTANT WARNING: All of the timelines in this article, and the existence of in-person interviews and appointments, have been placed into limbo by the coronavirus or COVID-19 epidemic. Although USCIS offices will receive applications by mail for processing, the agency is not scheduling any biometrics appointments, citizenship interviews, or swearing-in ceremonies as of March 2020.
The most important part of your citizenship application is Form N-400, Application for Naturalization, available for free download on the USCIS website. Along with the N-400 form, you will need to prepare and send:
Make a complete copy before mailing your application to USCIS, following the instructions on its website.
After accepting your application, USCIS will send you a notice telling you when and where to go to give "biometrics" (fingerprints). This will be the USCIS "application support center" or ASC closest to the address you gave on your application. If you need to change the date of your biometrics appointment, the notice tells you what to do.
Several months later, you will be notified when to appear for a personal interview at your local USCIS office. During that interview, a USCIS officer will review your application and approve or deny your citizenship.
You will need to show that you can speak and read in English. If you aren't already comfortable with this, taking a class at a local adult school may help. To focus on the key vocabulary, see the Study Materials for the English Test page of the USCIS website.
You will also need to study a list of questions in preparation for the exam on U.S. history and government. Unless you qualify for a waiver (based on age or disability), you will need to answer 60% of these correctly in order to pass the exam. See the Study Materials for the Civics Test page of the USCIS website to find these questions and various means of studying them.
At your interview, a USCIS officer will review your application and ask you various questions from Form N-400. This serves a dual purpose: to confirm your eligibility and to test your knowledge of spoken English. The officer will also ask you to write a sentence in English, and ask you questions from the list that you studied.
You will also need to answer six out of ten questions correctly concerning U.S. history and government, so as to pass the civics test. There are 100 total questions, and the officer will choose which ones to ask you.
If approved by USCIS, you will be scheduled for a swearing-in ceremony. You are not a citizen until you are sworn in.
If you are denied because of failure to pass the English or the civics test, you will be automatically rescheduled for another interview, to take place within 90 days. If you are not approved for some other reason, you may be given time to overcome the problem. This would be a good time to get a lawyer's help.
For detailed information on the citizenship application process, see Becoming a U.S. Citizen: A Guide to the Law, Exam & Interview, by Ilona Bray (Nolo).