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This will come either in the form of an email (if you filed online) or as a letter. File this in your records as proof you began the process. The USCIS will mail you a Form I-797C, or Notice of Action. This is the notice you should use as evidence that you filed this application. Again, this is the notice that lists the information you need for your subsequent appointment. Bring your appointment letter with you in addition to some form of photo identification. The biometrics appointment consists of fingerprinting and taking your photo for your green card. It is nothing to worry about unless you have a new and improved criminal record. If you need proof of documentation while the USCIS is reviewing your status, say so at your appointment. They will stamp your passport to indicate that you have filed for a new card. It will enable you to exit the U.S. and reenter. Once more, wait for notification from the United States Immigration Service concerning further appointments. Otherwise, the next step is receiving your card. There may be an in-person interview you will have to attend at a regional location. It's just as likely you may not have any additional appointments to attend and will receive your new green card in the mail.

Summary:
Wait for a notification of receipt from the USCIS. Attend your biometrics appointment. Review the checklist sent to you by the United States Immigration Service and gather your paperwork.